SUMMARY AND GENERAL CONCLUSIONS
The question of the origin of the adaptations with which the last three
chapters have so largely dealt is one of the most difficult problems in
the whole range of biology, and yet it is one whose immense interest has
tempted philosophers in the past, and will no doubt continue to excite
the imagination of biologists for many years to come. No pretence has
been made in the preceding pages to account for the cause of a single
useful variation. We have examined the evidence, and from this we
believe the assumption justified that such variations do sometimes
appear. The more fundamental question as to the origin of these
variations has not been taken up, except in those cases in which the
adaptive response appeared directly in connection with a known external
cause. But these kinds of responses do not appear to have been the
source of the other adaptations of the organic world. Our discussion has
been largely confined to the problem of the widespread occurrence of
adaptation in living things, and to the most probable kinds of known
variations that could have given rise to these adaptations. But, to
repeat, we have made no attempt to account for the causes or the origin
of the different kinds of variation.
Nägeli, in speaking of the methods of the earlier theorists in Germany,
remarks with much acumen: “We might have expected that after the period
of the Nature-philosophizers, which in Germany crippled the best forces
that might have been used for the advance of the science, we should have
learnt something from experience, and have carefully guarded the field
of real scientific work from philosophical speculation. But the outcome
has shown that, in general, the philosophical, philological, and
æsthetic expression always gets the upper hand, and a fundamental and
exact treatment of scientific questions remains limited to a small
circle. The public at large always shows a distinct preference for the
so-called idealistic, poetic, and speculative modes of expression.” The
truth of this statement can scarcely be doubted when in our own time we
have seen more than once the same method employed with great public
applause. Nowhere is this more apparent than in the writings of many of
the followers of Darwin in respect to the adaptations of living things.
To imagine that a particular organ is useful to its possessor, and to
account for its origin because of the imagined benefit conferred, is the
general procedure of the followers of this school. Although protests
have from time to time been raised against this unwarrantable way of
settling the matter, they have been largely ignored and forgotten. The
fallacy of the argument has, for example, been admirably pointed out by
Bateson in the following statement:[36] “In examining cases of variation
I have not thought it necessary to speculate on the usefulness or
harmfulness of the variations described. For reasons given in Section II
such speculation, whether applied to normal structures or to variation,
is barren and profitless. If any one is curious on these questions of
Adaptation, he may easily thus exercise his imagination. In any case of
Variation there are a hundred ways in which it may be beneficial or
detrimental. For instance, if the ‘hairy’ variety of the moor-hen became
established on an island, as many strange varieties have been, I do not
doubt that ingenious persons would invite us to see how the hairiness
fitted the bird in some special way for life in that island in
particular. Their contention would be hard to deny, for on this class of
speculation the only limitations are those of the ingenuity of the
author. While the only test of utility is the success of the organism,
even this does not indicate the utility of one part of the economy, but
rather the net fitness of the whole.” Keeping in mind the admonitions
contained in the two preceding quotations, let us pass in review and
attempt to analyze more fully the different points that have been
considered in the preceding chapters.
Footnote 36:
“Materials for the Study of Variation.”
It has been pointed out that the evidence in favor of the theory of
evolution appears to establish this theory with great probability,
although a closer examination shows that we are almost completely in the
dark as to how the process has come about. For example, we have not yet
been able to determine whether the great groups of animals and plants
owe their resemblance to descent from a single original species or from
a large number of species. The former view is more plausible, because on
it we appear to be furnished with a better explanation of resemblances
as due to divergence of character. Yet even here a closer scrutiny of
the homologies of comparative anatomy shows that this explanation may be
more apparent than real. If discontinuous variation represents the steps
by which evolution has taken place, the artificiality of the explanation
is apparent, at least to a certain degree.
Admitting that the theory of evolution is the most probable view that we
have to account for the facts, we next meet with two further
questions,—the origin of species and the meaning of adaptation. These
are two separate and distinct questions, and not one and the same as the
Darwinian theory claims. The fact that all organisms are more or less
adapted to live in some environment appears from our examination to have
no direct connection with the origin of the adaptation, for, in the
first place, it seems probable that, in general, organisms do not
respond adaptively to the environment and produce new species in this
way; and, in the second place, there is no evidence to show that
variation from internal causes is so regulated that only adaptive
structures arise (although only adaptive ones may survive).
Our general conclusion is then as follows: A species does not arise from
another one because it is better adapted. Selection, in other words,
does not account for the origin of new species; and adaptation cannot be
taken as the measure of a species.
It may sound like a commonplace to state that only those individuals
survive and propagate themselves that can find some place in nature
where they can exist and leave descendants; and yet this statement may
contain all that it is necessary to assume, in order to account for the
fact that organisms are, on the whole, adapted. Let us see how this view
differs from the Darwinian statement of the origination of new forms
through a process of natural selection.
According to Darwin’s view of the origin of species, each new species is
gradually formed out of an older one, because of the advantage that the
new individual may have over the parent form. Each step forward is
acquired, because it better adapts the organism to the old, or to a new
set of conditions. In contrast to this, I have urged that the formation
of the new species is, as a rule, quite independent of its adaptive
value in regard to the parent species. But after it has appeared, its
survival will depend upon whether it can find a place in nature where it
can exist and leave descendants. If it should be well adapted to an
environment, it will be represented in it by a large number of
individuals. If it is poorly adapted, it may only barely succeed in
existing, and leave correspondingly fewer descendants. If its
adaptiveness falls below a certain point, it can never get a permanent
foothold, however often it may appear. Thus the test of survival
determines which species can remain in existence, and which cannot, but
new species are not manufactured in this way. How far subsequent
variations may be supposed to be determined by the survival of certain
species and the destruction of others will be discussed presently.
The difference between the two points of view that we are contrasting
can be best brought out by considering the two other kinds of selection
which Darwin supposes to have been at work; namely, artificial and
sexual selection.
Darwin thinks that the results of artificial selection are brought about
by the breeder picking out fluctuating variations. It appears that he
has probably overestimated the extent to which this process can be
carried; for while there can be no doubt that a certain standard, or
fixity of type, can be obtained by selecting fluctuating variations, yet
it now seems quite certain that the extent to which this can be carried
is very limited. It appears that other factors have also played an
important rôle; amongst these the occasional appearance of discontinuous
variation, also the bringing under cultivation of the numerous “smaller
species” of De Vries, or the so-called “single variations” of Darwin.
Further, the effects of intercrossing in all combinations of the above
forms of variations, followed by the selection of certain of the new
forms obtained, has been largely employed, and also the direct influence
of food and of other external conditions, which may be necessary to keep
the race up to a certain standard, have played a part in some cases. The
outcome is, therefore, by no means so simple as one might infer from
Darwin’s treatment of the subject in his “Origin of Species.” For these
reasons, as well as for others that have been given, it will be evident
that the process of artificial selection cannot be expected to give a
very clear idea of how natural selection could act.
It is, however, the process of sexual selection that brings out in the
strongest contrast the difference between Darwin’s main idea of natural
selection and the law of the survival of species. In sexual selection
the competition is supposed to be always between the individuals of the
same species and of the same sex. There can be no doubt in one’s mind,
after reading “The Descent of Man,” that Darwin held firmly to the
belief that the individual differences, or fluctuating variations,
furnish the material for selection. In this way it could never happen
that two competing species could exterminate each other, because in the
one the males were better adorned, or killed each other off on a larger
scale, owing to the presence of special weapons of warfare. It is clear
that on the law of the survival of species, secondary sexual characters
cannot be supposed to have evolved because of their value. Their origin
is totally inexplicable on this view. In fact, the presence of the
ornaments must be in some cases injurious to the existence of the
species. The interpretation of this means, I think, that individual
competition cannot be as severe as Darwin believed, and cannot lead to
the results that he imagined it does. For this reason it seemed
important to make as careful an examination of the claims of the theory
of sexual selection as possible, and I hope that the outcome of the
examination has shown quite definitely that the theory is incompetent to
account for the facts that it claims to explain. It is certain in this
case that we are dealing with a phenomenon that must be studied quite
apart from any selective value that the secondary sexual organs may
have. If this is granted, it will be seen that there is here a wide
field for experimental investigation that is practically untouched.
It is evident that the first step that will clear the way to a fuller
understanding of the problem of evolution must be a more thorough
examination of the question of variation. Darwin himself fully
appreciated this fact, yet until within the last fifteen years the study
of variation has been largely neglected. With a fuller knowledge of the
nature of fluctuating variation as the outcome of the studies of Galton,
Pearson, De Vries, and others, and with a fuller knowledge of the
possibilities of discontinuous variation as emphasized by Bateson and by
De Vries, and, further, with a better knowledge of some of the laws of
inheritance in these cases, we have begun to get a different conception
of how evolution has come about. It may be well, therefore, to go once
more over the main points in regard to the different kinds of variation.
While it has been found that no two individuals of a species are exactly
alike, yet, taken as a group, the variations appear as though they
followed the law of chance. The descendants of the group show the same
differences. Thus the group _as a whole_ appears constant, while the
individuals fluctuate continually in all directions. This is what we
understand by fluctuating variation. If the external conditions are
changed, a new “_mode_” may appear, but the change is generally only a
temporary one, and lasts only as long as the new conditions remain.
Thus, while the direct influence of the environment may show for a time,
the result is transient. Even if it were permanent, there is no evidence
that the adaptation of organisms could be accounted for in this way
unless the response were useful. It appears that this sometimes really
occurs, especially in responses to temperature, to moisture, to the
amount of salts in solution, to poisonous substances, etc. In this way,
one kind of adaptation is brought about, but there is no evidence that
the great number of structural adaptations have thus arisen.
The Lamarckian principle of the inheritance of acquired characters has
also been supposed by many writers to be an important source of adaptive
variation. An examination of this theory is not found to inspire
confidence. We have, therefore, eliminated this hypothesis on the ground
that it lacks evidence in its favor, and also because it appears
improbable that in this way many of the adaptations in organisms could
have been acquired.
Finally, there is the group of discontinuous variations. Of these there
may be several kinds, and there is some evidence showing that there are
such. For the present we may include all the different sorts under the
term _mutation_, meaning that the new character or group of characters
suddenly appears, and is inherited in its new form. From the results of
De Vries it appears that mutations are sometimes scattering, at least in
the case of the evening primrose. From such scattering mutations, the
smaller species or varieties (in so far as these do not depend on local
conditions) arise. There is here an important point of agreement with
Darwin’s idea in regard to evolution, inasmuch as he supposed that
varieties are incipient species. Our point of view is different,
however, in that we do not suppose these varieties (mutations) to have
been gradually formed out of fluctuating variations by a process of
selection, but to have arisen at once by a single mutation. It also
appears that in some cases a single new mutation may develop in a
species. We may suppose that the new form might in such a case supplant
the parent species by absorbing it, or both may go on living side by
side, as will be more likely the case if they are adapted to somewhat
different conditions.
A number of writers have supposed that evolution marches steadily
forward toward its final goal, which may even lead in some cases to the
final but inevitable destruction of the species. By certain writers this
view has been called orthogenesis, although at other times the idea is
not so much that there is advance in a straight line, as advance in all
directions. This appears to be Nägeli’s view. It gives a splendid
picture of the organic world, as irresistibly marching toward its
goal,—a relentless process in some cases, leading to final annihilation,
a beneficent process in other cases, leading to the fullest perfection
of form of which the type is capable. Compared with the vacillating
progress which is supposed to be the outcome of individual selection,
this view of progression has a grandeur that appeals directly to the
imagination. We must be guided, however, by evidence, rather than by
sentiment. The case will, moreover, bear closer scrutiny. If evolution
has indeed taken place by the survival of a series of mutations, whose
origin has no connection with their value, does not this in the end
amount to nearly the same thing as maintaining that evolution of
organisms has been a steady progress forward,—a progress undirected by
external forces, but the outcome of internal development? Admitting that
innumerable creations have been lopped off, because they could find no
foothold, yet, as Nägeli points out, the result is that, instead of a
dense tangle of forms, there has been left relatively few that have been
found capable of existing,—those that have found some place in which
they can live and leave progeny. From this point of view it may appear,
at first thought, that the idea of evolution through mutations involves
a fundamentally different view from that of the Darwinian school of
selection; but in so far as selection also depends on the spontaneous
appearance of fluctuating variations, the same point of view is to some
extent involved,—only the steps are supposed to be smaller. This point
is usually ignored and passed over in silence by the Darwinians, but, as
Wigand has pointed out, it makes very little difference whether the
stages in the process of evolution are imagined to be very small or
somewhat larger, so long as they are spontaneous. Selection does not do
more than determine the survival of what is offered to it, and does not
create anything new.
It is true that if the fluctuating variations that are selected be
connected by very slight differences with an almost continuous series of
other forms, and if little by little such a series be advanced in a
given direction by selection, we get the idea of a continuity, whose
advance is determined by selection. It is this conception that appears
to give the theory of natural selection a creative power, which in
reality it does not possess, and certainly not in the modified form in
which the theory was finally left by Darwin. For Darwin found himself
forced to admit that, unless a very considerable number of individuals
varied at the same time and in the same direction, the formation of new
species could not take place, and this idea of many individuals varying
at the same time, and in the same direction, at once involves the
conception that evolution moves forward by some force residing in the
organism, driving it forwards or backwards. Instability comes, perhaps,
nearer to expressing this idea than any other term, and yet to evolve
from a protozoan to a man implies the idea of something more than simple
unstableness.
The idea that Weismann has touched upon in this connection, namely, that
the survival of a given form determines the future course of evolution
for that form, is very plausible, and also fits in well with the results
of our experience in the field of the inheritance of variations. We see
new variations or mutations departing in some or in many characters from
the original type, apparently by new combinations or perturbations of
those already present. We never expect to see a bird emerge from the egg
of an alligator. Thus it appears that by the survival of certain forms
the future course of evolution is determined in so far as the new types
of mutation are thereby limited. Weismann means, however, that in this
way new plus or minus steps will be indefinitely determined amongst the
new fluctuating variations, but this statement is contradicted by our
experience of the results of artificial selection. The upper limit does
not keep on pushing out indefinitely in the direction determined by the
first selection, but is soon brought to a standstill. So that, as far as
Weismann’s hypothesis is concerned, the idea appears to have no special
value. On the other hand, this idea may be fruitful if applied to
mutations, but here unfortunately we have not sufficient experience to
guide us, and we do not know definitely whether a new character that
appears as a mutation will be more likely, in subsequent mutations, to
go on increasing in some of the descendants. Thus, while the mutation
theory must assume that some new characters will go on heaping up, we
lack the experimental evidence to show that this really occurs. It would
be also equally important to determine, whether, if after several
mutations have successively appeared in the same direction, there would
be an established tendency to go on in the same direction in some of the
future mutations. But here again we must wait until we have more data
before we attempt to build up a theory on such a basis.
The attacks on the Darwinian school by the followers of the modern
school of experimentalists are with few exceptions based on the
assumption that the natural selectionists pretend that their principle
is a sort of creative force,—a factor in evolution in the sense of being
an active agent. This assumption of the selectionists has led many of
them to ignore a fundamental weakness of their theory, namely, the
origin of the variations themselves, although Darwin did not overlook or
ignore this side of the problem, or fail to realize its importance, as
have some of his more ardent, but less critical, followers. They have
contented themselves, as a rule, with pointing out that certain
structures are useful, and this has seemed to them sufficient proof that
the structures must have been acquired because of their value. In
contrast to this complacency of the selectionists, we find here and
there naturalists who have, from time to time, insisted that the
scientific problem of evolution is not to be found in the principle of
selection, but in the origin of the variations themselves. It will be
clear, from what has been said, that this is our position also, and for
us adaptation itself does not appear to be any more a problem that can
be examined by scientific methods, than the lack of adaptation. The
causes of the change of whatever kind should be our immediate quest. The
destruction of the unfit, because they can find no place where they can
exist, does not explain the origin of the fit.
Over and beyond the primary question of the _origin_ of the adaptive, or
non-adaptive, structure is the fact that we find that the great majority
of animals and plants show distinct evidence of being suited or adapted
to live in a special environment, _i.e._ their structure and their
responses are such that they can live and leave descendants behind them.
I can see but two ways in which to account for this condition, either
(1) teleologically, by assuming that only adaptive variations arise, or
(2) by the survival of only those mutations that are sufficiently
adapted to get a foothold. Against the former view is to be urged that
the evidence shows quite clearly that variations (mutations) arise that
are not adaptive. On the latter view the dual nature of the problem that
we have to deal with becomes evident, for we assume that, while the
origin of the adaptive structures must be due to purely physical
principles in the widest sense, yet whether an organism that arises in
this way shall persist depends on whether it can find a suitable
environment. This latter is in one sense selection, although the word
has come to have a different significance, and, therefore, I prefer to
use the term _survival of species_.
The origin of a new form and its survival after it has appeared have
been often confused by the Darwinian school and have given the critics
of this school a fair chance for ridiculing the selection theory. The
Darwinian school has supposed that it could explain the origin of
adaptations on the basis of their usefulness. In this it seems to me
they are wrong. Their opponents, on the other hand, have, I believe,
gone too far when they state that the present condition of animals and
plants can be explained without applying the test of survival, or in a
broad sense the principle of selection amongst species.
It will be clear, therefore, in spite of the criticism that I have not
hesitated to apply to many of the phases of the selection theory,
especially in relation to the selection of the individuals of a species,
that I am not unappreciative of the great value of that part of Darwin’s
idea which claims that the _condition_ of the organic world, as we find
it, cannot be accounted for entirely without applying the principle of
selection in one form or another. This idea will remain, I think, a most
important contribution to the theory of evolution. We may sum up our
position categorically in the following statements:
Animals and plants are not changed in this or in that part in order to
become better adjusted to a given environment, as the Darwinian theory
postulates. Species exist that are in some respects very poorly adapted
to the environment in which they must live. If competition were as
severe as the selection theory assumes, this imperfection would not
exist.
In other cases a structure may be more perfect than the requirements of
selection demand. We must admit, therefore, that we cannot measure the
organic world by the measure of utility alone. If it be granted that
selection is not a moulding force in the organic world, we can more
easily understand how both less perfection and greater perfection may be
present than the demands of survival require.
If we suppose that new mutations and “definitely” inherited variations
suddenly appear, some of which will find an environment to which they
are more or less well fitted, we can see how evolution may have gone on
without assuming new species have been formed through a process of
competition. Nature’s supreme test is survival. She makes new forms to
bring them to this test through mutation, and does not remodel old forms
through a process of individual selection.
------------------------------------------------------------------------
INDEX
Acclimatization, 319.
Acorn, 24.
Acracids, 160.
Adaptation, definition of, 1.
Adjustments, individual, 12.
Agassiz, 1, 44, 61.
Agelæus, 173.
Alcohol, 13.
Algæ, red, 9.
Alkaloids, 13.
Allen, 173, 307-310.
Allolobophora, 380.
Alpheus, 344.
Ammophila, 5.
Ammotragus, 208.
Ampelopsis, 403.
Amphimixis, 448-449.
Amphioxus, 399.
Ancon race, 315-316.
Angiostomum, 422.
Anguillidæ, 320.
Annelids, 19, 20.
Anolis, 10, 194.
Ant-eater, 227, 228.
Antelope, 6, 206, 208.
Antitoxin, 14.
Ants, 141-146, 354, 386, 407.
Aphids, 384-386, 419, 426.
Apus, 418.
Archæopteryx, 41, 42, 53, 54.
Ardea, 200.
Argus pheasant, 199.
Aristolochia Clematitis, 10, 11, 12.
Arsenic, 13.
Artemia, 306.
Ascidians, 417.
Askenasy, 161.
Aspalax, 227.
Asterina, 421-422.
Autenrieth, 58.
Baboon, 209.
Bacteria, 14, 15, 111, 398.
Baer, Von, 60, 61, 74, 75.
Bamboo, 313.
Barnacles, 417.
Bartlett, 209, 220.
Bat, 2.
Bates, 183, 186.
Bateson, 273, 278, 453.
Beard, 210, 211, 216.
Beard, J., 435.
Bee, 2, 3, 19, 143, 179, 303, 350, 406, 420, 421, 425, 438.
Beethoven, 218.
Beetles, 182, 183, 189.
Bell-bird, 198.
Beneden, Van, 441.
Berbura goat, 208.
Biogenetic Law, 71.
Birds, 6, 407;
definition of group, 36;
evolution of, 41;
instincts of young, 4;
nest, 4;
of paradise, 6;
teeth of, 301;
variation in, 309-312.
Blind animals, 354.
Blow-fly, 383.
Bonellia, 353, 417.
Born, 424.
Bos, 206.
Boveri, 433.
Bovidæ, 207.
Branchipus, 306.
Brocadello, 428.
Brooks, 441.
Brown-Séquard, 232, 241, 250-257.
Buffon, 300.
Bull, 207, 315.
Bütschli, 441.
Butterfly, 3, 184, 389.
Cactus, 10.
Caffein, 13.
California salmon, 19.
Calkins, 443-447.
Callionymus, 191.
Calocalanus, 177.
Cameron, 425.
Canestrini, 178.
Canidæ, 308.
Canis, 410.
Carbonnier, 190, 192.
Cardamine, 335.
Cardinalis, 173.
Cardium, 305.
Cassowary, 202.
Castle, 148, 321, 435, 438.
Caterpillar, 5, 8, 186.
Cattle, 411.
Cats, 209.
Cercopithecus, 208.
Cervus, 304.
Chara, 420.
Charrin, 257.
Chick, 57, 406.
Child, 72.
Chinese pheasants, 6.
Chlorophyl, 9.
Cicadas, 187, 188.
Ciona, 148.
Classification, 31-36.
Classification, scheme of, 38.
Cockatoo, 6.
Colaptes, 310.
Colias, 185.
Colonial forms, 127.
Color, 18, 19, 24, 133, 375.
Coloration, 5, 6, 7, 23, 357-360.
Columba livia, 76.
Comb of bees, 4.
Communal marriages, 210.
Competition, 104, 112, 119, 120, 121, 122, 123.
Compositæ, 130.
Cones, 310.
Conklin, 72.
Cope, 49, 259.
Copridæ, 183.
Coral-snakes, 194.
Correlated variation, 94.
Correlation, 134.
Cottus, 191.
Crab, 15, 248, 344, 354.
Crickets, 188.
Crocodiles, 193.
Crosby, 398.
Cross-fertilization, 21.
Crossing of species, 148, 149, 150.
Crystal, 29.
Cryptocerus, 144.
Ctenophors, 417.
Cuckoo, 139, 140, 141.
Cuénot, 427-428, 435.
Culicidæ, 188.
Cunningham, 257-260.
Cuvier, 44, 301.
Cynocephalus, 209.
Cypridopsis, 392-393.
Cyprinodonts, 190.
Cypris, 320.
Dall, 260.
Dallinger, 320.
Danaids, 160.
Dances, 195.
Daphnia, 305, 418.
Darwin, C., numerous references throughout the text.
Darwin, Erasmus, 223.
Date-palm, 313.
Davenport, 264, 266, 321.
Dean, 358.
Death, 370.
Death, feigning, 410, 411.
Deer, 309.
Degeneration, 165.
Delamare, 257.
Descent theory, 31-35.
Desmarest, 206.
Desmodium, 403.
Dianthus, 149.
Didelphys, 410.
Dimorphism, 360.
Dingoes, 314.
Dinophilus, 428.
Diptera, 180, 188.
Divergence of character, 127, 128.
Dog, 226.
Draba, 288, 289, 290, 292, 294.
Draco, 194.
Dragonet, 191.
Drill, 209.
Ducks, 94, 314.
Düsing, 423.
Dutrochet, 320.
Earthworm, 380, 382, 383, 384, 417.
Echidna, 54.
Eciton, 144.
Egerton, 204.
Egg, 429-430, 432.
Eggs, number of, 19.
Egypt, animals of, 225.
Egyptian remains of animals, 43, 44.
Eimer, 158, 260.
Eisig, 72.
Electric organs, 22, 132, 372.
Elephant, 110-111, 206, 304.
Emu, 202.
Entoscolax, 353.
Epihippus, 50.
Equus, 50.
Eristales, 188.
Esmeralda, 182.
Euploids, 160.
Eustephanus, 201.
Evolution, 29.
Ewart, 238.
Exercise, 12.
External conditions, 130.
Eye, 13, 131, 132.
Eye, evolution of, 131, 132.
Eye, of flatfish, 137.
Fayal Islands, 124.
Felidæ, 308.
Felis, 308.
Fish, change of color, 16.
Fishes, 7.
Fishes, secondary sexual character of, 190.
Flatfish, 137, 138.
Flatworms, 417.
Fleischmann, 45-57.
Flounders, 228, 346, 347.
Flowers, 9, 17, 21, 342, 399, 422, 429.
Fly, 428.
Foot of horse, 47.
Forel, 5.
Fossil horses, 52.
Foxes, 210, 410.
Franqueiros cattle, 315.
Frogs, 193, 320, 382, 405.
Frogs, cross-fertilization, 150.
Fruit, down of, 133.
Fundulus, 16.
Galton, 236, 270-272, 289, 441.
Gavials, 301.
Geddes and Thompson, 417, 423, 426, 427.
Geer, De, 178.
Gegenbaur, 49.
Gelasimus, 177.
Geoffroy St.-Hilaire, 44, 67, 300-303.
Geological evidence, 39.
Gerbe, 429.
Germinal selection, 154.
Gibbon, 213.
Gill-clefts, 62, 63, 64, 73.
Giraffe, 6, 203, 229, 248-249.
Glacier, 28.
Glowworm, 23.
Goat, 206, 208.
Gonionema, 399.
Gorilla, 205.
Gothic period, 47, 48.
Gould, 197.
Graba, 124, 125.
Grafting, 153.
Grasshoppers, 8, 188.
Gray, 126.
Greyhound, 134.
Growth of plants, 17.
Guillemots, 124.
Guinea-pigs, 232.
Günther, 190.
Gymnotus, 132.
Haeckel, 48, 49, 56, 70, 71, 79, 80, 82.
Hartman, 187.
Heart, 66, 67.
Heliconids, 160.
Helix, 344, 345-346.
Hemiptera, 181.
Heredity, 270.
Hermaphroditic animals, 126.
Hertwig, O., 78, 79, 80, 81, 82, 83.
Hertwig, R., 447.
Hieracium, 330, 331.
Hildebrand, 148.
Hill, 252.
Hipparion, 51.
Hippeastrum, 148.
His, 71, 72.
Holmes, 72.
Hornbills, 219.
Horns, 229.
Horse, 42, 224.
Horse-chestnut, 24.
Hothura, 410.
Hottentots, 212.
Hudson, 140, 195, 409-412.
Humming-birds, 6, 197, 228.
Hurst, 75, 76, 77, 78.
Huxley, 49, 156, 233.
Hyatt, 259.
Hybrids, 149, 239.
Hydatina, 417.
Hydroides, 348.
Hylobates, 205.
Hymenoptera, 181.
Ice, 28.
Ichneumonidæ, 181.
Idioplasm, 335.
Immunity, 13.
India cattle, 208.
Infanticide, 25.
Inorganic adaptations, 26.
Insectivorous plants, 10.
Insects, coloration of, 7;
wingless, 228.
Instinct, 25, 139, 140, 141.
Irish elk, 247.
Jackson, 260.
Jaguar, 209.
Japanese cock, 163.
Jennings, 395.
Jonghe, 314.
Jordan, 292.
Joseph, 428.
Junco, 311.
Kallima, 7, 161, 162, 358.
Kangaroo, 229, 351.
Katydid, 8.
Kent, W. Saville, 191.
Kidneys, 66.
Kielmeyer, 58.
Kirby, 232.
Kiwi, 354.
Kölreuter, 149.
Korschelt, 428.
Labidocera, 393.
Lamarck, 146, 222-233.
Lamarckian factor, 94, 205, 211, 222, 458.
Lang, 345.
Law of Biogenesis, 30.
Leaf, resemblance to, 7.
Leaves, closing of, 11.
Leeches, 417.
Leguminosæ, 124.
Leidy, 46.
Length of life, 20.
Lenhossek, 435.
Leopard, 209.
Lepidoptera, 172.
Leptothrix, 320.
Leucophys, 442.
Lichen, 9.
Lillie, 72.
Limbs of vertebrates, 46.
Linaria, 401.
Linnæan species, 83.
Linnæus, 191.
Lion, 6.
Lizards, 7, 16, 17, 193.
Lobelia, 148.
Lobster, 343.
Lockwood, 138.
Locusts, 188.
Loeb, 383-392, 447.
Lomaria, 290.
Lowell lectures, 61.
Lumbriculus, 15.
Luminous organs, 133.
Lymnæa, 305, 322.
Lythrum, 363-370.
Machines, 26, 27, 28.
McIntosh, 176.
McNeill, 204.
Macropus, 192.
Malva, 401.
Mammalia, origin, 54, 202.
Man, 210.
Marsh, 49.
Matthews, 447.
Mauchamp, 315.
Maupas, 441, 442, 445.
May-flies, 19, 353, 389.
Mead, 72.
Meckel, 59, 60.
Melanism, 209.
Melospiza, 311.
Mendel, 278-286, 433, 436.
Mesohippus, 51.
Mimosa, 404.
Minnow, 16.
Minot, 433.
Mirabilis, 149, 150.
Mivart, 136, 137.
Mole, 1, 2, 18, 227.
Mole-cricket, 1, 2.
Molothrus, 140.
Monkeys, 207, 208.
Mons, Van, 332.
Montgomery, 432.
Moor-hen, 453.
Moquin-Tandon, 303.
Morton, Lord, 238.
Moschus, 206.
Moths, 184, 387, 388.
Moussu, 257.
Mozart, 218.
Mulberry, 313.
Müller, 182, 188.
Müller, Fritz, 148.
Muscles, 12.
Mycetes, 205.
Myzostomum, 422.
Nägeli, 161, 325-339, 459.
Natural selection, 104-107, 108, 109, 110, etc.;
definition of, 117.
Nauplius, 69.
Nectar, 124.
Nectar-feeding insects, 126, 127.
Nectarines, 134.
Negroes, 212.
Nematode, number of eggs, 110.
Nematus, 425.
Nemertian worms, 176.
Neo-Lamarckians, 240, 259-260.
Nepenthes, 10.
Nephela, 178.
Nest of birds, 4, 407-408.
Neuters, 142.
Nicotine, 13.
Nostocs, 320.
Notochord, 64, 65.
Nussbaum, 424.
Ocneria, 428.
Œnothera, 294-297.
Oken, 56, 58.
Old age, 21, 25.
Onites, 232.
Onychodromus, 442.
Opossum, 410.
Organs of little use, 22.
“Origin of Species,” 129.
Ornithorynchus, 54.
Orobanchia, 330.
Osborn, 259.
Oscillaria, 320.
Ostrich, 203, 354.
Oxalis, 290, 404.
Oxen, 304.
Oxide, 29.
Packard, 231, 260.
Paludina, 320, 322.
Pangenesis, 233-240.
Papilio, 158, 360, 388;
polyxenes, 3.
Paradisea, 197.
Paramæcium, 395-398, 442-447.
Parasitism, 352-353.
Parker, 393.
Parrots, 6.
Partridge, 410.
Passerella, 311.
Passiflora, 148.
Pavo, 317.
Peach, 134.
Peacock, 200, 317-318.
Peafowl, 198.
Pearson, 265, 267, 268-270.
Peas, 281-286.
Peckham, 178, 408.
Pelobates, 421.
Pflüger, 424, 430.
Phosphorescent organs, 22, 133.
Physa, 320, 322.
Pigeons, selection in, 102.
Pipilo, 311.
Pisum, 278.
Pithecia, 208.
Planaria, 380.
Planarians, 394.
Plants, 403, 415;
color of, 24;
influence of light, 17.
Plato, 304.
Plover, 202.
Poisons, 13, 14, 15, 18, 20, 377.
Polar bear, 6.
Pollen, 2, 125.
Polygon, 262.
Porthesia, 389.
Primula, 361-365.
Prionidæ, 182.
Probosces of insects, 127.
Protective coloration, 5, 6, 16, 158, 159.
Proteus, 227.
Protohippus, 51.
Przibram, 347.
Psyche, 419.
Ptarmigan, 5.
Pyrodes, 182.
Quetelet, 289.
Quiscalus, major, 173.
Rabbit, Porto Santo, 316-317.
Rabbits, 304.
Rabbits in Australia, 112.
Race-horse, 134.
Ranunculus, 305.
Ray-florets, 135.
Rays, electric organs of, 22.
Réaumur, 388.
Recapitulation theory, 58-83.
Reduction division, 432-433.
Regeneration, 15, 16, 27, 379.
Regulations, 27, 28.
Reproductive organs, 19.
Reptiles, fossil, 52, 53.
Rengger, 205.
Rhododendron, 330.
Rhynchæa, 201.
Riley, 424.
Rivers, 28.
Robinia, 404.
Romanes, 132, 250-256, 412.
Rose, 307.
Rothert, 398.
Rotifers, 118, 353, 424.
Roulin, 304.
Roundworms, 176, 353.
Rudimentary organs, 22.
Ryder, 260.
Sacculina, 353.
Sachs, 10.
Salmon, 19.
Salter, 314.
Salvin, 201.
Saphirina, 176.
Savages, 210.
Saw-flies, 425.
Scarlet tanager, 198.
Schaefer, 244.
Sclater, 198.
Scops, 312.
Scott, 148, 259.
Sea-anemone, 341.
Sea-urchin, 341.
Secondary sexual characters, 21.
Selection, 116.
Selection, artificial, 91, 92, 96, 97, 98.
Self-fertilization, 126.
Semper, 260.
Setchel, 320.
Sexual characters, secondary, 372-374.
Sexual selection, 167.
Sharp, 350, 425.
Sheep, 208.
Sherrington, 244.
Shrew mice, 206.
Silkworm, 428, 447.
Silver-bill, 410.
Sirex, 181.
Siricidæ, 181.
Sitaria, 194.
Skin, thickening of, 12, 13.
Skull, 37, 65.
Skunk, 3.
Slaves of ants, 141.
Sleep in plants, 404.
Sloth, 229.
Snail, 417.
Snails, color of, 23.
Snakes, 14, 193-194, 227.
Snowy owl, 6.
Solenobia, 419.
Soles, 137, 228.
Sparassus, 178.
Sparrow, 200;
English, 112.
Species, 31, 32, 33;
adaptation for good of, 19;
sharp separation of, 131.
Spencer, 240-246, 247, 290.
Spermatozoa, 150, 430-433.
Sphinx, 186, 388.
Spiders, 177-178, 179, 406;
web, 3.
Spirogyra, 420.
Spontaneous variability, 134.
Spores, 322.
Squilla, 177.
Squirrels, 210.
Stag-beetle, 179.
Stags, 203-204, 219.
Sterility, 147-152.
Strasburger, 395.
Stridulating organs, 188, 189.
Struggle for existence, 109, 110.
Stylonychia, 442.
Survival of the fittest, 107, 108, 109, 117.
Sutton, 432.
Swallow, 115.
Sweating, 12.
Tadpole, 321, 428.
Tail, 2.
Tanager, 6.
Tapeworm, 353;
number of eggs, 110.
Taraxacum, 305.
Tear-sacs, 206.
Teeth, bird’s, 67, 68.
Telegony, 95, 234, 237, 238, 239.
Tenthredinidæ, 181, 425.
Termite, number of eggs, 110.
Termitidæ, 350.
Thrush, 115.
Tipulæ, 188.
Toad, 7.
Torpedo, 132.
Towle, 392.
Transitional forms, 42.
Transmutation theory, 31, 34.
Traquair, 138.
Treadwell, 72.
Treat, 424.
Tree-frogs, 7.
Trichina, 353.
Trifolium, 404.
Triton, 193.
Turkeys, 314.
Turnix, 201, 202.
Turtles, 193.
Umbelliferæ, 135.
Uria lacrymans, 124.
Utricularia, 10,
Vanessa, 360.
Variability, 92, 93, 95, 96, 318-319.
Variation, 261, 340.
Variation, fluctuating, 100, 118, 123.
Variation under domestication, 136.
Varieties, 106, 107, 148.
Varigny, De, 303-306, 314-315, 322.
Venus fly-trap, 9.
Verbascum, 148, 149.
Vertebrates, evolution of, 40, 45.
Vilmorin, 303, 314.
Vinson, 178.
Vries, De, 97, 278, 289-298, 340.
Vulpine, 209.
Wallace, 7, 162, 186, 202, 221, 249.
Walrus, 203.
Walsh, 181.
Walther, 59.
Wasp, 3, 5, 408, 409.
Waterton, 198.
Web, spider’s, 3, 4.
Weir, 171.
Weismann, 154-166, 441, 448-450.
Westwood, 188.
Whale, 227, 301.
Wilson, E. B., 72.
Wing of bat, 2.
Wolf, 308, 376.
Wolves, 412.
Women, 210.
Woodpecker, 228.
Wounds, healing of, 15.
Yarrell, 138.
Yung, 424, 436.
Zebu cattle, 208.
Zeleny, 348.
Zoea, 69, 70.
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Transcriber’s note:
○ Missing or obscured punctuation was silently corrected.
○ Typographical errors were silently corrected.
○ Inconsistent spelling and hyphenation were made consistent only
when a predominant form was found in this book.
○ The cover image was created by the transcriber and is placed in
the public domain.
○ Some character-based illusrations were re-drawn using different
characters.