Recapitulation of the difficulties on the theory of
Natural Selection -- Recapitulation of the general and special circumstances
in its favour -- Causes of the general belief in the immutability of species
-- How far the theory of natural selection may be extended -- Effects of its
adoption on the study of Natural history -- Concluding remarks.
As this whole volume is one long argument, it may be
convenient to the reader to have the leading facts and inferences briefly
recapitulated.
2 That many and grave objections may be advanced
against the theory of descent with modification through natural selection, I
do not deny. I have endeavoured to give to them their full force. Nothing at
first can appear more difficult to believe than that the more complex organs
and instincts should have been perfected, not by means superior to, though
analogous with, human reason, but by the accumulation of innumerable slight
variations, each good for the individual possessor. Nevertheless, this
difficulty, though appearing to our imagination insuperably great, cannot be
considered real if we admit the following propositions, namely,--that
gradations in the perfection of any organ or instinct, which we may consider,
either do now exist or could have existed, each good of its kind,--that all
organs and instincts are, in ever so slight a degree, variable,--and, lastly,
that there is a struggle for existence leading to the preservation of each
profitable deviation of structure or instinct. The truth of these propositions
cannot, I think, be disputed.
3 It is, no doubt, extremely difficult even to
conjecture by what gradations many structures have been perfected, more
especially amongst broken and failing groups of organic beings; but we see so
many strange gradations in nature, as is proclaimed by the canon, 'Natura non
facit saltum,' that we ought to be extremely cautious in saying that any organ
or instinct, or any whole being, could not have arrived at its present state
by many graduated steps. There are, it must be admitted, cases of special
difficulty on the theory of natural selection; and one of the most curious of
these is the existence of two or three defined castes of workers or sterile
females in the same community of ants; but I have attempted to show how this
difficulty can be mastered.
4 With respect to the almost universal sterility of
species when first crossed, which forms so remarkable a contrast with the
almost universal fertility of varieties when crossed, I must refer the reader
to the recapitulation of the facts given at the end of the eighth chapter,
which seem to me conclusively to show that this sterility is no more a special
endowment than is the incapacity of two trees to be grafted together, but that
it is incidental on constitutional differences in the reproductive systems of
the intercrossed species. We see the truth of this conclusion in the vast
difference in the result, when the same two species are crossed reciprocally;
that is, when one species is first used as the father and then as the mother.
5 The fertility of varieties when intercrossed and of
their mongrel offspring cannot be considered as universal; nor is their very
general fertility surprising when we remember that it is not likely that
either their constitutions or their reproductive systems should have been
profoundly modified. Moreover, most of the varieties which have been
experimentised on have been produced under domestication; and as domestication
apparently tends to eliminate sterility, we ought not to expect it also to
produce sterility.
6 The sterility of hybrids is a very different case
from that of first crosses, for their reproductive organs are more or less
functionally impotent; whereas in first crosses the organs on both sides are
in a perfect condition. As we continually see that organisms of all kinds are
rendered in some degree sterile from their constitutions having been disturbed
by slightly different and new conditions of life, we need not feel surprise at
hybrids being in some degree sterile, for their constitutions can hardly fail
to have been disturbed from being compounded of two distinct organisations.
This parallelism is supported by another parallel, but directly opposite,
class of facts; namely, that the vigour and fertility of all organic beings
are increased by slight changes in their conditions of life, and that the
offspring of slightly modified forms or varieties acquire from being crossed
increased vigour and fertility. So that, on the one hand, considerable changes
in the conditions of life and crosses between greatly modified forms, lessen
fertility; and on the other hand, lesser changes in the conditions of life and
crosses between less modified forms, increase fertility.
7 Turning to geographical distribution, the
difficulties encountered on the theory of descent with modification are grave
enough. All the individuals of the same species, and all the species of the
same genus, or even higher group, must have descended from common parents; and
therefore, in however distant and isolated parts of the world they are now
found, they must in the course of successive generations have passed from some
one part to the others. We are often wholly unable even to conjecture how this
could have been effected. Yet, as we have reason to believe that some species
have retained the same specific form for very long periods, enormously long as
measured by years, too much stress ought not to be laid on the occasional wide
diffusion of the same species; for during very long periods of time there will
always be a good chance for wide migration by many means. A broken or
interrupted range may often be accounted for by the extinction of the species
in the intermediate regions. It cannot be denied that we are as yet very
ignorant of the full extent of the various climatal and geographical changes
which have affected the earth during modern periods; and such changes will
obviously have greatly facilitated migration. As an example, I have attempted
to show how potent has been the influence of the Glacial period on the
distribution both of the same and of representative species throughout the
world. We are as yet profoundly ignorant of the many occasional means of
transport. With respect to distinct species of the same genus inhabiting very
distant and isolated regions, as the process of modification has necessarily
been slow, all the means of migration will have been possible during a very
long period; and consequently the difficulty of the wide diffusion of species
of the same genus is in some degree lessened.
8 As on the theory of natural selection an interminable
number of intermediate forms must have existed, linking together all the
species in each group by gradations as fine as our present varieties, it may
be asked, Why do we not see these linking forms all around us? Why are not all
organic beings blended together in an inextricable chaos? With respect to
existing forms, we should remember that we have no right to expect (excepting
in rare cases) to discover directly connecting links between them, but only
between each and some extinct and supplanted form. Even on a wide area, which
has during a long period remained continuous, and of which the climate and
other conditions of life change insensibly in going from a district occupied
by one species into another district occupied by a closely allied species, we
have no just right to expect often to find intermediate varieties in the
intermediate zone. For we have reason to believe that only a few species are
undergoing change at any one period; and all changes are slowly effected. I
have also shown that the intermediate varieties which will at first probably
exist in the intermediate zones, will be liable to be supplanted by the allied
forms on either hand; and the latter, from existing in greater numbers, will
generally be modified and improved at a quicker rate than the intermediate
varieties, which exist in lesser numbers; so that the intermediate varieties
will, in the long run, be supplanted and exterminated.
9 On this doctrine of the extermination of an
infinitude of connecting links, between the living and extinct inhabitants of
the world, and at each successive period between the extinct and still older
species, why is not every geological formation charged with such links? Why
does not every collection of fossil remains afford plain evidence of the
gradation and mutation of the forms of life? We meet with no such evidence,
and this is the most obvious and forcible of the many objections which may be
urged against my theory. Why, again, do whole groups of allied species appear,
though certainly they often falsely appear, to have come in suddenly on the
several geological stages? Why do we not find great piles of strata beneath
the Silurian system, stored with the remains of the progenitors of the
Silurian groups of fossils? For certainly on my theory such strata must
somewhere have been deposited at these ancient and utterly unknown epochs in
the world's history.
10 I can answer these questions and grave objections
only on the supposition that the geological record is far more imperfect than
most geologists believe. It cannot be objected that there has not been time
sufficient for any amount of organic change; for the lapse of time has been so
great as to be utterly inappreciable by the human intellect. The number of
specimens in all our museums is absolutely as nothing compared with the
countless generations of countless species which certainly have existed. We
should not be able to recognise a species as the parent of any one or more
species if we were to examine them ever so closely, unless we likewise
possessed many of the intermediate links between their past or parent and
present states; and these many links we could hardly ever expect to discover,
owing to the imperfection of the geological record. Numerous existing doubtful
forms could be named which are probably varieties; but who will pretend that
in future ages so many fossil links will be discovered, that naturalists will
be able to decide, on the common view, whether or not these doubtful forms are
varieties? As long as most of the links between any two species are unknown,
if any one link or intermediate variety be discovered, it will simply be
classed as another and distinct species. Only a small portion of the world has
been geologically explored. Only organic beings of certain classes can be
preserved in a fossil condition, at least in any great number. Widely ranging
species vary most, and varieties are often at first local,--both causes
rendering the discovery of intermediate links less likely. Local varieties
will not spread into other and distant regions until they are considerably
modified and improved; and when they do spread, if discovered in a geological
formation, they will appear as if suddenly created there, and will be simply
classed as new species. Most formations have been intermittent in their
accumulation; and their duration, I am inclined to believe, has been shorter
than the average duration of specific forms. Successive formations are
separated from each other by enormous blank intervals of time; for
fossiliferous formations, thick enough to resist future degradation, can be
accumulated only where much sediment is deposited on the subsiding bed of the
sea. During the alternate periods of elevation and of stationary level the
record will be blank. During these latter periods there will probably be more
variability in the forms of life; during periods of subsidence, more
extinction.
11 With respect to the absence of fossiliferous
formations beneath the lowest Silurian strata, I can only recur to the
hypothesis given in the ninth chapter. That the geological record is imperfect
all will admit; but that it is imperfect to the degree which I require, few
will be inclined to admit. If we look to long enough intervals of time,
geology plainly declares that all species have changed; and they have changed
in the manner which my theory requires, for they have changed slowly and in a
graduated manner. We clearly see this in the fossil remains from consecutive
formations invariably being much more closely related to each other, than are
the fossils from formations distant from each other in time.
12 Such is the sum of the several chief objections and
difficulties which may justly be urged against my theory; and I have now
briefly recapitulated the answers and explanations which can be given to them.
I have felt these difficulties far too heavily during many years to doubt
their weight. But it deserves especial notice that the more important
objections relate to questions on which we are confessedly ignorant; nor do we
know how ignorant we are. We do not know all the possible transitional
gradations between the simplest and the most perfect organs; it cannot be
pretended that we know all the varied means of Distribution during the long
lapse of years, or that we know how imperfect the Geological Record is. Grave
as these several difficulties are, in my judgment they do not overthrow the
theory of descent with modification.
13 Now let us turn to the other side of the argument.
Under domestication we see much variability. This seems to be mainly due to
the reproductive system being eminently susceptible to changes in the
conditions of life; so that this system, when not rendered impotent, fails to
reproduce offspring exactly like the parent-form. Variability is governed by
many complex laws,--by correlation of growth, by use and disuse, and by the
direct action of the physical conditions of life. There is much difficulty in
ascertaining how much modification our domestic productions have undergone;
but we may safely infer that the amount has been large, and that modifications
can be inherited for long periods. As long as the conditions of life remain
the same, we have reason to believe that a modification, which has already
been inherited for many generations, may continue to be inherited for an
almost infinite number of generations. On the other hand we have evidence that
variability, when it has once come into play, does not wholly cease; for new
varieties are still occasionally produced by our most anciently domesticated
productions.
14 Man does not actually produce variability; he only
unintentionally exposes organic beings to new conditions of life, and then
nature acts on the organisation, and causes variability. But man can and does
select the variations given to him by nature, and thus accumulate them in any
desired manner. He thus adapts animals and plants for his own benefit or
pleasure. He may do this methodically, or he may do it unconsciously by
preserving the individuals most useful to him at the time, without any thought
of altering the breed. It is certain that he can largely influence the
character of a breed by selecting, in each successive generation, individual
differences so slight as to be quite inappreciable by an uneducated eye. This
process of selection has been the great agency in the production of the most
distinct and useful domestic breeds. That many of the breeds produced by man
have to a large extent the character of natural species, is shown by the
inextricable doubts whether very many of them are varieties or aboriginal
species.
15 There is no obvious reason why the principles which
have acted so efficiently under domestication should not have acted under
nature. In the preservation of favoured individuals and races, during the
constantly-recurrent Struggle for Existence, we see the most powerful and
ever-acting means of selection. The struggle for existence inevitably follows
from the high geometrical ratio of increase which is common to all organic
beings. This high rate of increase is proved by calculation, by the effects of
a succession of peculiar seasons, and by the results of naturalisation, as
explained in the third chapter. More individuals are born than can possibly
survive. A grain in the balance will determine which individual shall live and
which shall die,--which variety or species shall increase in number, and which
shall decrease, or finally become extinct. As the individuals of the same
species come in all respects into the closest competition with each other, the
struggle will generally be most severe between them; it will be almost equally
severe between the varieties of the same species, and next in severity between
the species of the same genus. But the struggle will often be very severe
between beings most remote in the scale of nature. The slightest advantage in
one being, at any age or during any season, over those with which it comes
into competition, or better adaptation in however slight a degree to the
surrounding physical conditions, will turn the balance.
16 With animals having separated sexes there will in
most cases be a struggle between the males for possession of the females. The
most vigorous individuals, or those which have most successfully struggled
with their conditions of life, will generally leave most progeny. But success
will often depend on having special weapons or means of defence, or on the
charms of the males; and the slightest advantage will lead to victory.
17 As geology plainly proclaims that each land has
undergone great physical changes, we might have expected that organic beings
would have varied under nature, in the same way as they generally have varied
under the changed conditions of domestication. And if there be any variability
under nature, it would be an unaccountable fact if natural selection had not
come into play. It has often been asserted, but the assertion is quite
incapable of proof, that the amount of variation under nature is a strictly
limited quantity. Man, though acting on external characters alone and often
capriciously, can produce within a short period a great result by adding up
mere individual differences in his domestic productions; and every one admits
that there are at least individual differences in species under nature. But,
besides such differences, all naturalists have admitted the existence of
varieties, which they think sufficiently distinct to be worthy of record in
systematic works. No one can draw any clear distinction between individual
differences and slight varieties; or between more plainly marked varieties and
sub-species, and species. Let it be observed how naturalists differ in the
rank which they assign to the many representative forms in Europe and North
America.
18 If then we have under nature variability and a
powerful agent always ready to act and select, why should we doubt that
variations in any way useful to beings, under their excessively complex
relations of life, would be preserved, accumulated, and inherited? Why, if man
can by patience select variations most useful to himself, should nature fail
in selecting variations useful, under changing conditions of life, to her
living products? What limit can be put to this power, acting during long ages
and rigidly scrutinising the whole constitution, structure, and habits of each
creature,--favouring the good and rejecting the bad? I can see no limit to
this power, in slowly and beautifully adapting each form to the most complex
relations of life. The theory of natural selection, even if we looked no
further than this, seems to me to be in itself probable. I have already
recapitulated, as fairly as I could, the opposed difficulties and objections:
now let us turn to the special facts and arguments in favour of the theory.
19 On the view that species are only strongly marked and
permanent varieties, and that each species first existed as a variety, we can
see why it is that no line of demarcation can be drawn between species,
commonly supposed to have been produced by special acts of creation, and
varieties which are acknowledged to have been produced by secondary laws. On
this same view we can understand how it is that in each region where many
species of a genus have been produced, and where they now flourish, these same
species should present many varieties; for where the manufactory of species
has been active, we might expect, as a general rule, to find it still in
action; and this is the case if varieties be incipient species. Moreover, the
species of the large genera, which afford the greater number of varieties or
incipient species, retain to a certain degree the character of varieties; for
they differ from each other by a less amount of difference than do the species
of smaller genera. The closely allied species also of the larger genera
apparently have restricted ranges, and they are clustered in little groups
round other species--in which respects they resemble varieties. These are
strange relations on the view of each species having been independently
created, but are intelligible if all species first existed as varieties.
20 As each species tends by its geometrical ratio of
reproduction to increase inordinately in number; and as the modified
descendants of each species will be enabled to increase by so much the more as
they become more diversified in habits and structure, so as to be enabled to
seize on many and widely different places in the economy of nature, there will
be a constant tendency in natural selection to preserve the most divergent
offspring of any one species. Hence during a long-continued course of
modification, the slight differences, characteristic of varieties of the same
species, tend to be augmented into the greater differences characteristic of
species of the same genus. New and improved varieties will inevitably supplant
and exterminate the older, less improved and intermediate varieties; and thus
species are rendered to a large extent defined and distinct objects. Dominant
species belonging to the larger groups tend to give birth to new and dominant
forms; so that each large group tends to become still larger, and at the same
time more divergent in character. But as all groups cannot thus succeed in
increasing in size, for the world would not hold them, the more dominant
groups beat the less dominant. This tendency in the large groups to go on
increasing in size and diverging in character, together with the almost
inevitable contingency of much extinction, explains the arrangement of all the
forms of life, in groups subordinate to groups, all within a few great
classes, which we now see everywhere around us, and which has prevailed
throughout all time. This grand fact of the grouping of all organic beings
seems to me utterly inexplicable on the theory of creation.
21 As natural selection acts solely by accumulating
slight, successive, favourable variations, it can produce no great or sudden
modification; it can act only by very short and slow steps. Hence the canon of
'Natura non facit saltum,' which every fresh addition to our knowledge tends
to make more strictly correct, is on this theory simply intelligible. We can
plainly see why nature is prodigal in variety, though niggard in innovation.
But why this should be a law of nature if each species has been independently
created, no man can explain.
22 Many other facts are, as it seems to me, explicable
on this theory. How strange it is that a bird, under the form of woodpecker,
should have been created to prey on insects on the ground; that upland geese,
which never or rarely swim, should have been created with webbed feet; that a
thrush should have been created to dive and feed on sub-aquatic insects; and
that a petrel should have been created with habits and structure fitting it
for the life of an auk or grebe! and so on in endless other cases. But on the
view of each species constantly trying to increase in number, with natural
selection always ready to adapt the slowly varying descendants of each to any
unoccupied or ill-occupied place in nature, these facts cease to be strange,
or perhaps might even have been anticipated.
23 As natural selection acts by competition, it adapts
the inhabitants of each country only in relation to the degree of perfection
of their associates; so that we need feel no surprise at the inhabitants of
any one country, although on the ordinary view supposed to have been specially
created and adapted for that country, being beaten and supplanted by the
naturalised productions from another land. Nor ought we to marvel if all the
contrivances in nature be not, as far as we can judge, absolutely perfect; and
if some of them be abhorrent to our ideas of fitness. We need not marvel at
the sting of the bee causing the bee's own death; at drones being produced in
such vast numbers for one single act, and being then slaughtered by their
sterile sisters; at the astonishing waste of pollen by our fir-trees; at the
instinctive hatred of the queen bee for her own fertile daughters; at
ichneumonidae feeding within the live bodies of caterpillars; and at other
such cases. The wonder indeed is, on the theory of natural selection, that
more cases of the want of absolute perfection have not been observed.
24 The complex and little known laws governing variation
are the same, as far as we can see, with the laws which have governed the
production of so-called specific forms. In both cases physical conditions seem
to have produced but little direct effect; yet when varieties enter any zone,
they occasionally assume some of the characters of the species proper to that
zone. In both varieties and species, use and disuse seem to have produced some
effect; for it is difficult to resist this conclusion when we look, for
instance, at the logger-headed duck, which has wings incapable of flight, in
nearly the same condition as in the domestic duck; or when we look at the
burrowing tucutucu, which is occasionally blind, and then at certain moles,
which are habitually blind and have their eyes covered with skin; or when we
look at the blind animals inhabiting the dark caves of America and Europe. In
both varieties and species correlation of growth seems to have played a most
important part, so that when one part has been modified other parts are
necessarily modified. In both varieties and species reversions to long-lost
characters occur. How inexplicable on the theory of creation is the occasional
appearance of stripes on the shoulder and legs of the several species of the
horse-genus and in their hybrids! How simply is this fact explained if we
believe that these species have descended from a striped progenitor, in the
same manner as the several domestic breeds of pigeon have descended from the
blue and barred rock-pigeon!
25 On the ordinary view of each species having been
independently created, why should the specific characters, or those by which
the species of the same genus differ from each other, be more variable than
the generic characters in which they all agree? Why, for instance, should the
colour of a flower be more likely to vary in any one species of a genus, if
the other species, supposed to have been created independently, have
differently coloured flowers, than if all the species of the genus have the
same coloured flowers? If species are only well-marked varieties, of which the
characters have become in a high degree permanent, we can understand this
fact; for they have already varied since they branched off from a common
progenitor in certain characters, by which they have come to be specifically
distinct from each other; and therefore these same characters would be more
likely still to be variable than the generic characters which have been
inherited without change for an enormous period. It is inexplicable on the
theory of creation why a part developed in a very unusual manner in any one
species of a genus, and therefore, as we may naturally infer, of great
importance to the species, should be eminently liable to variation; but, on my
view, this part has undergone, since the several species branched off from a
common progenitor, an unusual amount of variability and modification, and
therefore we might expect this part generally to be still variable. But a part
may be developed in the most unusual manner, like the wing of a bat, and yet
not be more variable than any other structure, if the part be common to many
subordinate forms, that is, if it has been inherited for a very long period;
for in this case it will have been rendered constant by long-continued natural
selection.
26 Glancing at instincts, marvellous as some are, they
offer no greater difficulty than does corporeal structure on the theory of the
natural selection of successive, slight, but profitable modifications. We can
thus understand why nature moves by graduated steps in endowing different
animals of the same class with their several instincts. I have attempted to
show how much light the principle of gradation throws on the admirable
architectural powers of the hive-bee. Habit no doubt sometimes comes into play
in modifying instincts; but it certainly is not indispensable, as we see, in
the case of neuter insects, which leave no progeny to inherit the effects of
long-continued habit. On the view of all the species of the same genus having
descended from a common parent, and having inherited much in common, we can
understand how it is that allied species, when placed under considerably
different conditions of life, yet should follow nearly the same instincts; why
the thrush of South America, for instance, lines her nest with mud like our
British species. On the view of instincts having been slowly acquired through
natural selection we need not marvel at some instincts being apparently not
perfect and liable to mistakes, and at many instincts causing other animals to
suffer.
27 If species be only well-marked and permanent
varieties, we can at once see why their crossed offspring should follow the
same complex laws in their degrees and kinds of resemblance to their
parents,--in being absorbed into each other by successive crosses, and in
other such points,--as do the crossed offspring of acknowledged varieties. On
the other hand, these would be strange facts if species have been
independently created, and varieties have been produced by secondary laws.
28 If we admit that the geological record is imperfect
in an extreme degree, then such facts as the record gives, support the theory
of descent with modification. New species have come on the stage slowly and at
successive intervals; and the amount of change, after equal intervals of time,
is widely different in different groups. The extinction of species and of
whole groups of species, which has played so conspicuous a part in the history
of the organic world, almost inevitably follows on the principle of natural
selection; for old forms will be supplanted by new and improved forms. Neither
single species nor groups of species reappear when the chain of ordinary
generation has once been broken. The gradual diffusion of dominant forms, with
the slow modification of their descendants, causes the forms of life, after
long intervals of time, to appear as if they had changed simultaneously
throughout the world. The fact of the fossil remains of each formation being
in some degree intermediate in character between the fossils in the formations
above and below, is simply explained by their intermediate position in the
chain of descent. The grand fact that all extinct organic beings belong to the
same system with recent beings, falling either into the same or into
intermediate groups, follows from the living and the extinct being the
offspring of common parents. As the groups which have descended from an
ancient progenitor have generally diverged in character, the progenitor with
its early descendants will often be intermediate in character in comparison
with its later descendants; and thus we can see why the more ancient a fossil
is, the oftener it stands in some degree intermediate between existing and
allied groups. Recent forms are generally looked at as being, in some vague
sense, higher than ancient and extinct forms; and they are in so far higher as
the later and more improved forms have conquered the older and less improved
organic beings in the struggle for life. Lastly, the law of the long endurance
of allied forms on the same continent,--of marsupials in Australia, of
edentata in America, and other such cases,--is intelligible, for within a
confined country, the recent and the extinct will naturally be allied by
descent.
29 Looking to geographical distribution, if we admit
that there has been during the long course of ages much migration from one
part of the world to another, owing to former climatal and geographical
changes and to the many occasional and unknown means of dispersal, then we can
understand, on the theory of descent with modification, most of the great
leading facts in Distribution. We can see why there should be so striking a
parallelism in the distribution of organic beings throughout space, and in
their geological succession throughout time; for in both cases the beings have
been connected by the bond of ordinary generation, and the means of
modification have been the same. We see the full meaning of the wonderful
fact, which must have struck every traveller, namely, that on the same
continent, under the most diverse conditions, under heat and cold, on mountain
and lowland, on deserts and marshes, most of the inhabitants within each great
class are plainly related; for they will generally be descendants of the same
progenitors and early colonists. On this same principle of former migration,
combined in most cases with modification, we can understand, by the aid of the
Glacial period, the identity of some few plants, and the close alliance of
many others, on the most distant mountains, under the most different climates;
and likewise the close alliance of some of the inhabitants of the sea in the
northern and southern temperate zones, though separated by the whole
intertropical ocean. Although two areas may present the same physical
conditions of life, we need feel no surprise at their inhabitants being widely
different, if they have been for a long period completely separated from each
other; for as the relation of organism to organism is the most important of
all relations, and as the two areas will have received colonists from some
third source or from each other, at various periods and in different
proportions, the course of modification in the two areas will inevitably be
different.
30 On this view of migration, with subsequent
modification, we can see why oceanic islands should be inhabited by few
species, but of these, that many should be peculiar. We can clearly see why
those animals which cannot cross wide spaces of ocean, as frogs and
terrestrial mammals, should not inhabit oceanic islands; and why, on the other
hand, new and peculiar species of bats, which can traverse the ocean, should
so often be found on islands far distant from any continent. Such facts as the
presence of peculiar species of bats, and the absence of all other mammals, on
oceanic islands, are utterly inexplicable on the theory of independent acts of
creation.
31 The existence of closely allied or representative
species in any two areas, implies, on the theory of descent with modification,
that the same parents formerly inhabited both areas; and we almost invariably
find that wherever many closely allied species inhabit two areas, some
identical species common to both still exist. Wherever many closely allied yet
distinct species occur, many doubtful forms and varieties of the same species
likewise occur. It is a rule of high generality that the inhabitants of each
area are related to the inhabitants of the nearest source whence immigrants
might have been derived. We see this in nearly all the plants and animals of
the Galapagos archipelago, of Juan Fernandez, and of the other American
islands being related in the most striking manner to the plants and animals of
the neighbouring American mainland; and those of the Cape de Verde archipelago
and other African islands to the African mainland. It must be admitted that
these facts receive no explanation on the theory of creation.
32 The fact, as we have seen, that all past and present
organic beings constitute one grand natural system, with group subordinate to
group, and with extinct groups often falling in between recent groups, is
intelligible on the theory of natural selection with its contingencies of
extinction and divergence of character. On these same principles we see how it
is, that the mutual affinities of the species and genera within each class are
so complex and circuitous. We see why certain characters are far more
serviceable than others for classification;--why adaptive characters, though
of paramount importance to the being, are of hardly any importance in
classification; why characters derived from rudimentary parts, though of no
service to the being, are often of high classificatory value; and why
embryological characters are the most valuable of all. The real affinities of
all organic beings are due to inheritance or community of descent. The natural
system is a genealogical arrangement, in which we have to discover the lines
of descent by the most permanent characters, however slight their vital
importance may be.
33 The framework of bones being the same in the hand of
a man, wing of a bat, fin of the porpoise, and leg of the horse,--the same
number of vertebrae forming the neck of the giraffe and of the elephant,--and
innumerable other such facts, at once explain themselves on the theory of
descent with slow and slight successive modifications. The similarity of
pattern in the wing and leg of a bat, though used for such different
purpose,--in the jaws and legs of a crab,--in the petals, stamens, and pistils
of a flower, is likewise intelligible on the view of the gradual modification
of parts or organs, which were alike in the early progenitor of each class. On
the principle of successive variations not always supervening at an early age,
and being inherited at a corresponding not early period of life, we can
clearly see why the embryos of mammals, birds, reptiles, and fishes should be
so closely alike, and should be so unlike the adult forms. We may cease
marvelling at the embryo of an air-breathing mammal or bird having branchial
slits and arteries running in loops, like those in a fish which has to breathe
the air dissolved in water, by the aid of well-developed branchiae.
34 Disuse, aided sometimes by natural selection, will
often tend to reduce an organ, when it has become useless by changed habits or
under changed conditions of life; and we can clearly understand on this view
the meaning of rudimentary organs. But disuse and selection will generally act
on each creature, when it has come to maturity and has to play its full part
in the struggle for existence, and will thus have little power of acting on an
organ during early life; hence the organ will not be much reduced or rendered
rudimentary at this early age. The calf, for instance, has inherited teeth,
which never cut through the gums of the upper jaw, from an early progenitor
having well-developed teeth; and we may believe, that the teeth in the mature
animal were reduced, during successive generations, by disuse or by the tongue
and palate having been fitted by natural selection to browse without their
aid; whereas in the calf, the teeth have been left untouched by selection or
disuse, and on the principle of inheritance at corresponding ages have been
inherited from a remote period to the present day. On the view of each organic
being and each separate organ having been specially created, how utterly
inexplicable it is that parts, like the teeth in the embryonic calf or like
the shrivelled wings under the soldered wing-covers of some beetles, should
thus so frequently bear the plain stamp of inutility! Nature may be said to
have taken pains to reveal, by rudimentary organs and by homologous
structures, her scheme of modification, which it seems that we wilfully will
not understand.
35 I have now recapitulated the chief facts and
considerations which have thoroughly convinced me that species have changed,
and are still slowly changing by the preservation and accumulation of
successive slight favourable variations. Why, it may be asked, have all the
most eminent living naturalists and geologists rejected this view of the
mutability of species? It cannot be asserted that organic beings in a state of
nature are subject to no variation; it cannot be proved that the amount of
variation in the course of long ages is a limited quantity; no clear
distinction has been, or can be, drawn between species and well-marked
varieties. It cannot be maintained that species when intercrossed are
invariably sterile, and varieties invariably fertile; or that sterility is a
special endowment and sign of creation. The belief that species were immutable
productions was almost unavoidable as long as the history of the world was
thought to be of short duration; and now that we have acquired some idea of
the lapse of time, we are too apt to assume, without proof, that the
geological record is so perfect that it would have afforded us plain evidence
of the mutation of species, if they had undergone mutation.
36 But the chief cause of our natural unwillingness to
admit that one species has given birth to other and distinct species, is that
we are always slow in admitting any great change of which we do not see the
intermediate steps. The difficulty is the same as that felt by so many
geologists, when Lyell first insisted that long lines of inland cliffs had
been formed, and great valleys excavated, by the slow action of the
coast-waves. The mind cannot possibly grasp the full meaning of the term of a
hundred million years; it cannot add up and perceive the full effects of many
slight variations, accumulated during an almost infinite number of
generations.
37 Although I am fully convinced of the truth of the
views given in this volume under the form of an abstract, I by no means expect
to convince experienced naturalists whose minds are stocked with a multitude
of facts all viewed, during a long course of years, from a point of view
directly opposite to mine. It is so easy to hide our ignorance under such
expressions as the 'plan of creation,' 'unity of design,' &c., and to
think that we give an explanation when we only restate a fact. Any one whose
disposition leads him to attach more weight to unexplained difficulties than
to the explanation of a certain number of facts will certainly reject my
theory. A few naturalists, endowed with much flexibility of mind, and who have
already begun to doubt on the immutability of species, may be influenced by
this volume; but I look with confidence to the future, to young and rising
naturalists, who will be able to view both sides of the question with
impartiality. Whoever is led to believe that species are mutable will do good
service by conscientiously expressing his conviction; for only thus can the
load of prejudice by which this subject is overwhelmed be removed.
38 Several eminent naturalists have of late published
their belief that a multitude of reputed species in each genus are not real
species; but that other species are real, that is, have been independently
created. This seems to me a strange conclusion to arrive at. They admit that a
multitude of forms, which till lately they themselves thought were special
creations, and which are still thus looked at by the majority of naturalists,
and which consequently have every external characteristic feature of true
species,--they admit that these have been produced by variation, but they
refuse to extend the same view to other and very slightly different forms.
Nevertheless they do not pretend that they can define, or even conjecture,
which are the created forms of life, and which are those produced by secondary
laws. They admit variation as a vera causa in one case, they arbitrarily
reject it in another, without assigning any distinction in the two cases. The
day will come when this will be given as a curious illustration of the
blindness of preconceived opinion. These authors seem no more startled at a
miraculous act of creation than at an ordinary birth. But do they really
believe that at innumerable periods in the earth's history certain elemental
atoms have been commanded suddenly to flash into living tissues? Do they
believe that at each supposed act of creation one individual or many were
produced? Were all the infinitely numerous kinds of animals and plants created
as eggs or seed, or as full grown? and in the case of mammals, were they
created bearing the false marks of nourishment from the mother's womb?
Although naturalists very properly demand a full explanation of every
difficulty from those who believe in the mutability of species, on their own
side they ignore the whole subject of the first appearance of species in what
they consider reverent silence.
39 It may be asked how far I extend the doctrine of the
modification of species. The question is difficult to answer, because the more
distinct the forms are which we may consider, by so much the arguments fall
away in force. But some arguments of the greatest weight extend very far. All
the members of whole classes can be connected together by chains of
affinities, and all can be classified on the same principle, in groups
subordinate to groups. Fossil remains sometimes tend to fill up very wide
intervals between existing orders. Organs in a rudimentary condition plainly
show that an early progenitor had the organ in a fully developed state; and
this in some instances necessarily implies an enormous amount of modification
in the descendants. Throughout whole classes various structures are formed on
the same pattern, and at an embryonic age the species closely resemble each
other. Therefore I cannot doubt that the theory of descent with modification
embraces all the members of the same class. I believe that animals have
descended from at most only four or five progenitors, and plants from an equal
or lesser number.
40 Analogy would lead me one step further, namely, to
the belief that all animals and plants have descended from some one prototype.
But analogy may be a deceitful guide. Nevertheless all living things have much
in common, in their chemical composition, their germinal vesicles, their
cellular structure, and their laws of growth and reproduction. We see this
even in so trifling a circumstance as that the same poison often similarly
affects plants and animals; or that the poison secreted by the gall-fly
produces monstrous growths on the wild rose or oak-tree. Therefore I should
infer from analogy that probably all the organic beings which have ever lived
on this earth have descended from some one primordial form, into which life
was first breathed.
41 When the views entertained in this volume on the
origin of species, or when analogous views are generally admitted, we can
dimly foresee that there will be a considerable revolution in natural history.
Systematists will be able to pursue their labours as at present; but they will
not be incessantly haunted by the shadowy doubt whether this or that form be
in essence a species. This I feel sure, and I speak after experience, will be
no slight relief. The endless disputes whether or not some fifty species of
British brambles are true species will cease. Systematists will have only to
decide (not that this will be easy) whether any form be sufficiently constant
and distinct from other forms, to be capable of definition; and if definable,
whether the differences be sufficiently important to deserve a specific name.
This latter point will become a far more essential consideration than it is at
present; for differences, however slight, between any two forms, if not
blended by intermediate gradations, are looked at by most naturalists as
sufficient to raise both forms to the rank of species. Hereafter we shall be
compelled to acknowledge that the only distinction between species and
well-marked varieties is, that the latter are known, or believed, to be
connected at the present day by intermediate gradations, whereas species were
formerly thus connected. Hence, without quite rejecting the consideration of
the present existence of intermediate gradations between any two forms, we
shall be led to weigh more carefully and to value higher the actual amount of
difference between them. It is quite possible that forms now generally
acknowledged to be merely varieties may hereafter be thought worthy of
specific names, as with the primrose and cowslip; and in this case scientific
and common language will come into accordance. In short, we shall have to
treat species in the same manner as those naturalists treat genera, who admit
that genera are merely artificial combinations made for convenience. This may
not be a cheering prospect; but we shall at least be freed from the vain
search for the undiscovered and undiscoverable essence of the term species.
42 The other and more general departments of natural
history will rise greatly in interest. The terms used by naturalists of
affinity, relationship, community of type, paternity, morphology, adaptive
characters, rudimentary and aborted organs, &c., will cease to be
metaphorical, and will have a plain signification. When we no longer look at
an organic being as a savage looks at a ship, as at something wholly beyond
his comprehension; when we regard every production of nature as one which has
had a history; when we contemplate every complex structure and instinct as the
summing up of many contrivances, each useful to the possessor, nearly in the
same way as when we look at any great mechanical invention as the summing up
of the labour, the experience, the reason, and even the blunders of numerous
workmen; when we thus view each organic being, how far more interesting, I
speak from experience, will the study of natural history become!
43 A grand and almost untrodden field of inquiry will be
opened, on the causes and laws of variation, on correlation of growth, on the
effects of use and disuse, on the direct action of external conditions, and so
forth. The study of domestic productions will rise immensely in value. A new
variety raised by man will be a far more important and interesting subject for
study than one more species added to the infinitude of already recorded
species. Our classifications will come to be, as far as they can be so made,
genealogies; and will then truly give what may be called the plan of creation.
The rules for classifying will no doubt become simpler when we have a definite
object in view. We possess no pedigrees or armorial bearings; and we have to
discover and trace the many diverging lines of descent in our natural
genealogies, by characters of any kind which have long been inherited.
Rudimentary organs will speak infallibly with respect to the nature of
long-lost structures. Species and groups of species, which are called
aberrant, and which may fancifully be called living fossils, will aid us in
forming a picture of the ancient forms of life. Embryology will reveal to us
the structure, in some degree obscured, of the prototypes of each great class.
44 When we can feel assured that all the individuals of
the same species, and all the closely allied species of most genera, have
within a not very remote period descended from one parent, and have migrated
from some one birthplace; and when we better know the many means of migration,
then, by the light which geology now throws, and will continue to throw, on
former changes of climate and of the level of the land, we shall surely be
enabled to trace in an admirable manner the former migrations of the
inhabitants of the whole world. Even at present, by comparing the differences
of the inhabitants of the sea on the opposite sides of a continent, and the
nature of the various inhabitants of that continent in relation to their
apparent means of immigration, some light can be thrown on ancient geography.
45 The noble science of Geology loses glory from the
extreme imperfection of the record. The crust of the earth with its embedded
remains must not be looked at as a well-filled museum, but as a poor
collection made at hazard and at rare intervals. The accumulation of each
great fossiliferous formation will be recognised as having depended on an
unusual concurrence of circumstances, and the blank intervals between the
successive stages as having been of vast duration. But we shall be able to
gauge with some security the duration of these intervals by a comparison of
the preceding and succeeding organic forms. We must be cautious in attempting
to correlate as strictly contemporaneous two formations, which include few
identical species, by the general succession of their forms of life. As
species are produced and exterminated by slowly acting and still existing
causes, and not by miraculous acts of creation and by catastrophes; and as the
most important of all causes of organic change is one which is almost
independent of altered and perhaps suddenly altered physical conditions,
namely, the mutual relation of organism to organism,--the improvement of one
being entailing the improvement or the extermination of others; it follows,
that the amount of organic change in the fossils of consecutive formations
probably serves as a fair measure of the lapse of actual time. A number of
species, however, keeping in a body might remain for a long period unchanged,
whilst within this same period, several of these species, by migrating into
new countries and coming into competition with foreign associates, might
become modified; so that we must not overrate the accuracy of organic change
as a measure of time. During early periods of the earth's history, when the
forms of life were probably fewer and simpler, the rate of change was probably
slower; and at the first dawn of life, when very few forms of the simplest
structure existed, the rate of change may have been slow in an extreme degree.
The whole history of the world, as at present known, although of a length
quite incomprehensible by us, will hereafter be recognised as a mere fragment
of time, compared with the ages which have elapsed since the first creature,
the progenitor of innumerable extinct and living descendants, was created.
46 In the distant future I see open fields for far more
important researches. Psychology will be based on a new foundation, that of
the necessary acquirement of each mental power and capacity by gradation.
Light will be thrown on the origin of man and his history.
47 Authors of the highest eminence seem to be fully
satisfied with the view that each species has been independently created. To
my mind it accords better with what we know of the laws impressed on matter by
the Creator, that the production and extinction of the past and present
inhabitants of the world should have been due to secondary causes, like those
determining the birth and death of the individual. When I view all beings not
as special creations, but as the lineal descendants of some few beings which
lived long before the first bed of the Silurian system was deposited, they
seem to me to become ennobled. Judging from the past, we may safely infer that
not one living species will transmit its unaltered likeness to a distant
futurity. And of the species now living very few will transmit progeny of any
kind to a far distant futurity; for the manner in which all organic beings are
grouped, shows that the greater number of species of each genus, and all the
species of many genera, have left no descendants, but have become utterly
extinct. We can so far take a prophetic glance into futurity as to foretel
that it will be the common and widely-spread species, belonging to the larger
and dominant groups, which will ultimately prevail and procreate new and
dominant species. As all the living forms of life are the lineal descendants
of those which lived long before the Silurian epoch, we may feel certain that
the ordinary succession by generation has never once been broken, and that no
cataclysm has desolated the whole world. Hence we may look with some
confidence to a secure future of equally inappreciable length. And as natural
selection works solely by and for the good of each being, all corporeal and
mental endowments will tend to progress towards perfection.
48 It is interesting to contemplate an entangled bank,
clothed with many plants of many kinds, with birds singing on the bushes, with
various insects flitting about, and with worms crawling through the damp
earth, and to reflect that these elaborately constructed forms, so different
from each other, and dependent on each other in so complex a manner, have all
been produced by laws acting around us. These laws, taken in the largest
sense, being Growth with Reproduction; Inheritance which is almost implied by
reproduction; Variability from the indirect and direct action of the external
conditions of life, and from use and disuse; a Ratio of Increase so high as to
lead to a Struggle for Life, and as a consequence to Natural Selection,
entailing Divergence of Character and the Extinction of less-improved forms.
Thus, from the war of nature, from famine and death, the most exalted object
which we are capable of conceiving, namely, the production of the higher
animals, directly follows. There is grandeur in this view of life, with its
several powers, having been originally breathed into a few forms or into one;
and that, whilst this planet has gone cycling on according to the fixed law of
gravity, from so simple a beginning endless forms most beautiful and most
wonderful have been, and are being, evolved.
Subject Index
Aberrant groups
Abyssinia, plants of
Acclimatisation
Affinities of extinct species; of organic beings
Agassiz on Amblyopsis; on groups of species
suddenly appearing; on embryological succession; on the glacial period; on
embryological characters; on the embryos of vertebrata; on parallelism of
embryological development and geological succession
Algae of New Zealand
Alligators, males, fighting
Amblyopsis, blind fish
America, North, productions allied to those of
Europe; boulders and glaciers of; South, no modern formations on west coast
Ammonites, sudden extinction of
Anagallis, sterility of
Analog of variations
Ancylus
49 Animals, not domesticated from being variable;
domestic; descended from several stocks; acclimatisation of; of Australia;
with thicker fur in cold climates; blind, in caves; extinct, of Australia
Anomma
Antarctic islands, ancient flora of
Antirrhinum
Ants attending aphides; slave-making instinct
Ants, neuter, structure of
Aphides attended by ants
Aphis, development of
Apteryx
Arab horses
Aralo-Caspian Sea
Archiac, M. de, on the succession of species
Artichoke, Jerusalem
Ascension, plants of
Asclepias, pollen of
Asparagus
50 Aspicarpa
Asses, striped
Ateuchus
Audubon, on habits of frigate-bird; on variation in
birds'-nests; on heron eating seeds
Australia, animals of; dogs of; extinct animals of;
European plants in
Azara on flies destroying cattle
Azores, flora of
Babington, Mr., on British plants
Balancement of growth
Bamboo with hooks
Barberry, flowers of
Barrande, M., on Silurian colonies; on the
succession of species; on parallelism of palaeozoic formations; on
affinities of ancient species
Barriers, importance of
Batrachians on islands
Bats, how structure acquired; distribution of
51 Bear, catching water-insects
Bee, sting of; queen, killing rivals
Bees fertilizing flowers; hive; not sucking the red
clover; cell-making instinct; humble, cells of; parasitic
Beetles, wingless, in Madeira; with deficient tarsi
Bentham, Mr., on British plants; on classification
Berkeley, Mr., on seeds in salt-water
Bermuda, birds of
Birds acquiring fear; annually cross the Atlantic;
colour of, on continents; fossil, in caves of Brazil; of Madeira, Bermuda,
and Galapagos; song of males; transporting seeds; waders; wingless; with
traces of embryonic teeth
Bizcacha, affinities of
Bladder for swimming in fish
Blindness of cave animals
Blyth, Mr., on distinctness of Indian cattle; on
striped Hemionus; on crossed geese
Boar; shoulder-pad of
Borrow, Mr., on the Spanish pointer
52 Bory St. Vincent, on Batrachians
Bosquet, M., on fossil Chthamalus
Boulders, erratic, on the Azores
Branchiae
Brent, Mr., on house-tumblers; on hawks killing
pigeons
Brewer, Dr., on American cuckoo
Britain, mammals of
Bronn, on duration of specific forms
Brown, Robert, on classification
Buckman, on variation in plants
Buzareingues on sterility of varieties
Cabbage, varieties of, crossed
Calceolaria
Canary-birds, sterility of hybrids
Cape de Verde islands
53 Cape of Good Hope, plants of
Carrier-pigeons killed by hawks
Cassini on flowers of composita
Catasetum
Cats, with blue eyes, deaf; variation in habits of;
curling tail when going to spring
Cattle destroying fir-trees; destroyed by flies in
La Plata; breeds of, locally extinct; fertility of Indian and European
breeds
Cave, inhabitants of, blind
Centres of creation
Cephalopodae, development of
Cervulus
Cetacea, teeth and hair
Ceylon, plants of
Chalk formation
Charlock
Checks to increase; mutual
54 Chickens, instinctive tameness of
Chthamalinae
Chthamalus, cretacean species of
Circumstances favourable to selection of domestic
products; to natural selection
Cirripedes capable of crossing; carapace aborted;
their ovigerous frena; fossil; larvae of
Clift, Mr., on the succession of types
Climate, effects of, in checking increase of
beings; adaptation of, to organisms
Cobites, intestine of
Cockroach
Collections, palaeontological, poor
Colour, influenced by climate; in relation to
attacks by flies
Columba livia, parent of domestic pigeons
Colymbetes
Compositae, outer and inner florets of; male
flowers of
Conclusion, general
55 Conditions, slight changes in, favourable to
fertility
Coot
Coral-islands, seeds drifted to; reefs, indicating
movements of earth
Corn-crake
Correlation of growth in domestic productions; of
growth
Cowslip
Creation, single centres of
Crinum
Crosses, reciprocal
Crossing of domestic animals, importance in
altering breeds; advantages of; unfavourable to selection
Crustacea of New Zealand
Crustacean, blind
Cryptocerus
Ctenomys, blind
Cuckoo, instinct of
56 Currants, grafts of
Currents of sea, rate of
Cuvier on conditions of existence; on fossil
monkeys; Fred., on instinct
Dana, Prof., on blind cave-animals; on relations of
crustaceans of Japan, on crustaceans of New Zealand
De Candolle on struggle for existence; on
umbelliferae; on general affinities; Alph., on low plants, widely dispersed;
on widely-ranging plants being variable; on naturalisation; on winged seeds;
on Alpine species suddenly becoming rare; on distribution of plants with
large seeds; on vegetation of Australia; on fresh-water plants; on insular
plants
Degradation of coast-rocks
Denudation, rate of; of oldest rocks
Development of ancient forms
Devonian system
Dianthus, fertility of crosses
Dirt on feet of birds
Division, physiological, of labour
57 Dogs, hairless, with imperfect teeth; descended
from several wild stocks; domestic instincts of; inherited civilisation of;
fertility of breeds together; of crosses; proportions of, when young
Downing, Mr., on fruit-trees in America
Downs, North and South
Dragon-flies, intestines of
Drift-timber
Driver-ant
Drones killed by other bees
Duck, domestic, wings of, reduced; logger-headed
Duckweed
Dugong, affinities of
Dung-beetles with deficient tarsi
Dyticus
Earl, Mr., W., on the Malay Archipelago
Ears, drooping, in domestic animals; rudimentary
58 Earth, seeds in roots of trees
Eciton
Edentata, teeth and hair; fossil species of
Edwards, Milne, on physiological divisions of
labour; on gradations of structure; on embryological characters
Eggs, young birds escaping from
Electric organs
Elephant, rate increase; of glacial period
Existence, struggle for; conditions of
Eye, structure of; correction for aberration
Eyes reduced in moles
Fabre, M., on parasitic sphex
Falconer, Dr., on naturalisation of plants in
India; on fossil crocodile; on elephants and mastodons; and Cautley on
mammals of sub-Himalayan beds
Falkland Island, wolf of
Faults
Fear, instinctive, in birds
59 Feet of birds, young molluscs adhering to
Fertility of hybrids, from slight changes in
conditions; of crossed varieties
Fir-trees destroyed by cattle; pollen of
Fish, flying; teleostean, sudden appearance of;
eating seeds; fresh-water, distribution of
Fishes, ganoid, now confined to fresh water;
electric organs of; of southern hemisphere
Flight, powers of, how acquired
Flowers, structure of; in relation to crossing; of
composite and umbelliferae
Forbes, E., on colours of shells; on abrupt range
of shells in depth; on poorness of palaeontological collections; on
continuous succession of genera; on continental extensions; on distribution
during glacial period; on parallelism in time and space
Forests, changes in, in America
Formation, Devonian
Formations, thickness of, in Britain; intermittent
Formica rufescens; sanguinea; flava, neuter of
Frena, ovigerous, of cirripedes
Fresh-water productions, dispersal of
Fries on species in large genera being closely
allied to other species
60 Frigate-bird
Frogs on islands
Fruit-trees, gradual improvement of; in United
States; varieties of, acclimatised in United States
Fuci, crossed
Fur, thicker in cold climates
Furze
Galapagos Archipelago, birds of; productions of
Galeopithecus
Game, increase of, checked by vermin
Gartner on sterility of hybrids; on reciprocal
crosses; on crossed maize and verbascum; on comparison of hybrids and
mongrels
Geese, fertility when crossed; upland
Genealogy important in classification
Geoffroy St. Hilaire on balancement; on homologous
organs; Isidore, on variability of repeated parts; on correlation in
monstrosities; on correlation; on variable parts being often monstrous
Geographical distribution
Geology, future progress of
Giraffe, tail of
61 Glacial period
Gmelin on Distribution
Gnathodon, fossil
Godwin-Austin, Mr., on the Malay Archipelago
Goethe on compensation of growth
Gooseberry, grafts of
Gould, Dr. A., on land-shells; Mr., on colours of
birds; on birds of the Galapagos; on distribution of genera of birds
Gourds, crossed
Grafts, capacity of
Grasses, varieties of
Gray, Dr. Asa, on trees of United States; on
naturalised plants in the United States; on rarity of intermediate
varieties; on Alpine plants; Dr. J.E., on striped mule
Grebe
Grouse, colours of; red, a doubtful species
Growth, compensation of; correlation of, in
domestic products; correlation of
Habit, effect of, under domestication; effect of,
under nature; diversified, of same species
62 Hair and teeth, correlated
Harcourt, Mr. E.V., on the birds of Madeira
Hartung, M., on boulders in the Azores
Hazel-nuts
Hearne on habits of bears
Heath, changes in vegetation
Heer, O., on plants of Madeira
Helix pomatia
Helosciadium
Hemionus, striped
Herbert, W., on struggle for existence; on
sterility of hybrids
Hermaphrodites crossing
Heron eating seed
Heron, Sir R., on peacocks
63 Heusinger, on white animals not poisoned by certain
plants
Hewitt, Mr., on sterility of first crosses
Himalaya, glaciers of; plants of
Hippeastrum
Holly-trees, sexes of
Hollyhock, varieties of, crossed
Hooker, Dr., on trees of New Zealand
Hooker, Dr., on acclimatisation of Himalayan trees;
on flowers of umbelliferae; on glaciers of Himalaya; on algae of New
Zealand; on vegetation at the base of the Himalaya; on plants of Tierra del
Fuego; on Australian plants; on relations of flora of South America; on
flora of the Antarctic lands; on the plants of the Galapagos
Hooks on bamboos; to seeds on islands
Horner, Mr., on the antiquity of Egyptians
Horns, rudimentary
Horse, fossil, in La Plata
Horses destroyed by flies in La Plata; striped;
proportions of, when young
Horticulturists, selection applied by
64 Huber on cells of bees; P., on reason blended with
instinct; on habitual nature of instincts; on slave-making ants; on Melipona
domestica
Humble-bees; cells of
Hunter, J., on secondary sexual characters
Hutton, Captain, on crossed geese
Huxley, Prof., on structure of hermaphrodites; on
embryological succession; on homologous organs; on the development of aphis
Hybrids and mongrels compared
Hybridism
Hydra, structure of
Ibla
Icebergs transporting seeds
Individuals, numbers favourable to selection; many,
whether simultaneously created
Insects, colour of, fitted for habitation; sea-side
colours of; blind, in caves; luminous; neuter
Instincts, domestic
Intercrossing, advantages of
65 Japan, productions of
Java, plants of
Jones, Mr. J.M., on the birds of Bermuda
Jussieu on classification
Kentucky, caves of
Kerguelen-land, flora of
Kidney-bean, acclimatisation of
Kidneys of birds
Kirby on tarsi deficient in beetles
Knight, Andrew, on cause of variation
Kolreuter on the barberry; on sterility of hybrids;
on reciprocal crosses; on crossed varieties of nicotiana; on crossing male
and hermaphrodite flowers
Land-shells, distribution of, of Madeira,
naturalised
Languages, classification of
Lapse, great, of time
Larvae
66 Laurel, nectar secreted by the leaves
Laws of variation
Leech, varieties of
Leguminosae, nectar secreted by glands
Lepidosiren
Life, struggle for
Lingula, Silurian
Linnaeus, aphorism of
Lion, mane of; young of, striped
Lobelia fulgens
Lobelia, sterility of crosses
Loess of the Rhine
Lowness of structure connected with variability
Lowness, related to wide distribution
Lubbock, Mr., on the nerves of coccus
Lucas, Dr. P., on inheritance; on resemblance of
child to parent
67 Lund and Clausen on fossils of Brazil
Lyell, Sir C., on the struggle for existence; on
modern changes of the earth; on measure of denudation; on a carboniferous
land-shell; on fossil whales; on strata beneath Silurian system; on the
imperfection of the geological record; on the appearance of species; on
Barrande's colonies; on tertiary formations of Europe and North America; on
parallelism of tertiary formations; on transport of seeds by icebergs; on
great alternations of climate; on the distribution of fresh-water shells; on
land-shells of Madeira
Lyell and Dawson on fossilized trees in Nova Scotia
Macleay on analogical characters
Madeira, plants of; beetles of, wingless; fossil
land-shells of; birds of
Magpie tame in Norway
Maize, crossed
Malay Archipelago compared with Europe; mammals of
Malpighiaceae
Mammae, rudimentary
Mammals, fossil, in secondary formation; insular
Man, origin of races of
Manatee, rudimentary nails of
Marsupials of Australia, fossil species of
68 Martens, M., experiment on seeds
Martin, Mr. W.C., on striped mules
Matteuchi on the electric organs of rays
Matthiola, reciprocal crosses of
Means of dispersal
Melipona domestica
Metamorphism of oldest rocks
Mice destroying bees; acclimatisation of
Migration, bears on first appearance of fossils
Miller, Prof., on the cells of bees
Mirabilis, crosses of
Missel-thrush
Misseltoe, complex relations of
Mississippi, rate of deposition at mouth
Mocking-thrush of the Galapagos
Modification of species, how far applicable
69Moles, blind
Mongrels, fertility and sterility of; and hybrids
compared
Monkeys, fossil
Monocanthus
Mons, Van, on the origin of fruit-trees
Mozart, musical powers of
Mud, seeds in
Mules, striped
Muller, Dr. F., on Alpine Australian plants
Murchison, Sir, R., on the formations of Russia; on
azoic formations; on extinction
Mustela vison
Myanthus
Myrmecocystus
Myrmica, eyes of
Nails, rudimentary
Natural history, future progress of
70 Naturalisation of forms distinct from the
indigenous species; in New Zealand
Nautilus, Silurian
Nectar of plants
Nectaries, how formed
Nelumbium luteum
Nests, variation in
Neuter insects
New Zealand, productions of, not perfect;
naturalised products of; fossil birds of; glacial action in; crustaceans of;
algae of; number of plants of; flora of
Nicotiana, crossed varieties of; certain species
very sterile
Noble, Mr., on fertility of Rhododendron
Nodules, phosphatic, in azoic rocks
Oak, varieties of
Onites apelles
Orchis, pollen of
Ornithorhynchus
71 Ostrich not capable of flight; habit of laying eggs
together; American, two species of
Otter, habits of, how acquired
Ouzel, water
Owen, Prof., on birds not flying; on vegetative
repetition; on variable length of arms in ourang-outang; on the swim-bladder
of fishes; on electric organs; on fossil horse of La Plata; on relations of
ruminants and pachyderms; on fossil birds of New Zealand; on succession of
types; on affinities of the dugong; on homologous organs; on the
metamorphosis of cephalopods and spiders
Pacific Ocean, faunas of
Paley, on no organ formed to give pain
Pallas, on the fertility of the wild stocks of
domestic animals
Paraguay, cattle destroyed by flies
Parasites
Partridge, dirt on feet
Parts greatly developed, variable, degrees of
utility of
Parus major
Passiflora
Peaches in United States
Pear, grafts of
72 Pelargonium, flowers of, sterility of
Peloria
Pelvis of women
Period, glacial
Petrels, habits of
Phasianus, fertility of hybrids
Pheasant, young, wild
Pictet, Prof., on groups of species suddenly
appearing; on rate of organic change; on continuous succession of genera; on
close alliance of fossils in consecutive formations; on embryological
succession
Pierce, Mr., on varieties of wolves
Pigeons with feathered feet and skin between toes;
breeds described, and origin of; breeds of, how produced; tumbler, not being
able to get out of egg; reverting to blue colour; instinct of tumbling;
carriers, killed by hawks; young of
Pistil, rudimentary
Plants, poisonous, not affecting certain coloured
animals; selection applied to; gradual improvement of; not improved in
barbarous countries; destroyed by insects; in midst of range, have to
struggle with other plants; nectar of; fleshy, on sea-shores; fresh-water,
distribution of; low in scale; widely distributed
Plumage, laws of change in sexes of birds
Plums in the United States
73 Pointer dog, origin of; habits of
Poison not affecting certain coloured animals
Poison, similar effect of, on animals and plants
Pollen of fir-trees
Poole, Col., on striped hemionus
Prestwich, Mr., on English and French eocene
formation
Primrose; sterility of
Primula, varieties of
Proteolepas
Proteus
Psychology, future progress of
Quagga, striped
Quince, grafts of
Rabbit, disposition of young
Race-horses, Arab; English
Races, domestic, characters of
74 Ramond on plants of Pyrenees
Ramsay, Prof., on thickness of the British
formations; on faults
Ratio of increase
Rats, supplanting each other; acclimatisation of;
blind in cave
Rattle-snake
Reason and instinct
Recapitulation, general
Reciprocity of crosses
Record, geological, imperfect
Rengger on flies destroying cattle
Resemblance to parents in mongrels and hybrids
Reversion, law of inheritance
Rhododendron, sterility of
Richard, Prof., on Aspicarpa
Richardson, Sir J., on structure of squirrels; on
fishes of the southern hemisphere
Robinia, grafts of
75 Rodents, blind
Rudimentary organs
Rudiments important for classification
Sageret on grafts
Salmons, males fighting, and hooked jaws of
Salt-water, how far injurious to seeds
Saurophagus sulphuratus
Schiodte on blind insects
Schlegel on snakes
Sea-water, how far injurious to seeds
Sebright, Sir J., on crossed animals; on selection
of pigeons
Sedgwick, Prof., on groups of species suddenly
appearing
Seedlings destroyed by insects
Seeds, nutriment in; winged; power of resisting
salt-water; in crops and intestines of birds; eaten by fish; in mud; hooked,
on islands
Selection of domestic products, principle not of
recent origin; unconscious; natural; sexual; natural, circumstances
favourable to
76 Sexes, relations of
Sexual characters variable; selection
Sheep, Merino, their selection; two sub-breeds
unintentionally produced; mountain, varieties of
Shells, colours of, littoral, seldom embedded;
fresh-water, dispersal of; of Madeira; land, distribution of
Silene, fertility of crosses
Silliman, Prof., on blind rat
Skulls of young mammals
Slave-making instinct
Smith, Col. Hamilton, on striped horses; Mr. Fred.,
on slave-making ants; Mr., of Jordan Hill, on degradation of coast-rocks
Snap-dragon
Somerville, Lord, on selection of sheep
Sorbus, grafts of
Spaniel, King Charles's breed
Species, groups of, suddenly appearing; beneath
Silurian formations; successively appearing; changing simultaneously
throughout the world
Spencer, Lord, on increase in size of cattle
77 Sphex, parasitic
Spiders, development of
Spitz-dog crossed with fox
Sports in plants
Sprengel, C.C., on crossing; on ray-florets
Squirrels, gradations in structure
Staffordshire, heath, changes in
Stag-beetles, fighting
Sterility from changed conditions of life; of
hybrids; laws of; causes of; from unfavourable conditions; of certain
varieties
St. Helena, productions of
St. Hilaire, Aug., on classification
St. John, Mr., on habits of cats
Sting of bee
Stocks, aboriginal, of domestic animal
Strata, thickness of, in Britain
78 Stripes of horses
Swallow, one species supplanting another
Swim-bladder
Tail of giraffe; of aquatic animals; rudimentary
Tarsi deficient
Tausch on umbelliferous flowers
Teeth and hair correlated; embryonic, traces of, in
birds; rudimentary, in embryonic calf
Tegetmeier, Mr., on cells of bees
Temminck on distribution aiding classification
Thouin on grafts
Thrush, aquatic species of; mocking, of the
Galapagos; young of, spotted; nest of
Thuret, M., on crossed fuci
Thwaites, Mr., on acclimatisation
Tierra del Fuego, dogs of; plants of
Timber-drift
79 Time, lapse of
Titmouse
Toads on islands
Tobacco, crossed varieties of
Tomes, Mr., on the distribution of bats
Transitions in varieties
Trees on islands belong to peculiar orders; with
separated sexes
Trifolium pratense; incarnatum
Trigonia
Trilobites; sudden extinction of
Troglodytes
Tucutucu, blind
Tumbler pigeons, habits of, hereditary; young of
Turkey-cock, brush of hair on breast
Turkey, naked skin on head; young, wild
80 Turnip and cabbage, analogous variations of
Type, unity of
Types, succession of, in same areas
Udders enlarged by use; rudimentary
Ulex, young leaves of
Umbelliferae, outer and inner florets of
Unity of type
Use, effects of, under domestication
Utility, how far important in the construction of
each part
Valenciennes on fresh-water fish
Variability of mongrels and hybrids
Variations appear at corresponding ages; analogous
in distinct species
Varieties, natural; struggle between; domestic,
extinction of; transitional; when crossed, fertile; when crossed, sterile;
classification of
Verbascum, sterility of; varieties of, crossed
Verneuil, M. de, on the succession of species
81 Viola tricolor
Volcanic islands, denudation of
Vulture, naked skin on head
Wading-birds
Wallace, Mr., on origin of species; on law of
geographical distribution; on the Malay Archipelago
Wasp, sting of
Water, fresh, productions of
Water-hen
Waterhouse, Mr., on Australian marsupials; on
greatly developed parts being variable; on the cells of bees; on general
affinities
Water-ouzel
Watson, Mr. H.C., on range of varieties of British
plants; on acclimatisation; on flora of Azores; on Alpine plants; on rarity
of intermediate varieties
Weald, denudation of
Web of feet in water-birds
West Indian islands, mammals of
82 Westwood on species in large genera being closely
allied to others; on the tarsi of Engidae; on the antennae of hymenopterous
insects
Whales, fossil
Wheat, varieties of
White Mountains, flora of
Wings, reduction of size
Wings of insects homologous with branchiae;
rudimentary, in insects
Wolf crossed with dog; of Falkland Isles
Wollaston, Mr., on varieties of insects; on fossil
varieties of land-shells in Madeira; on colours of insects on sea-shore; on
wingless beetles; on rarity of intermediate varieties; on insular insects;
on land-shells of Madeira, naturalised
Wolves, varieties of
Woodpecker, habits of; green colour of
Woodward, Mr., on the duration of specific forms;
on the continuous succession of genera; on the succession of types
World, species changing simultaneously throughout
Wrens, nest of
Youatt, Mr., on selection, on sub-breeds of sheep;
on rudimentary horns in young cattle
83 Zebra, stripes on