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and if under the conditions to which they are exposed it had profited them
to ascend to a height, then the habit of slightly and irregularly revolving
might have been increased and utilized through natural selection, until
they had become converted into well-developed twining species.

With respect to the sensitiveness of the foot-stalks of the leaves and
flowers, and of tendrils, nearly the same remarks are applicable as in the
case of the revolving movements of twining plants. As a vast number of
species, belonging to widely distinct groups, are endowed with this kind of
sensitiveness, it ought to be found in a nascent condition in many plants
which have not become climbers. This is the case. I observed that the young
flower-peduncles of the above Maurandia curved themselves a little toward
the side which was touched. Morren found in several species of Oxalis that
the leaves and their foot-stalks moved, especially after exposure to a hot
sun, when they were gently and repeatedly touched, or when the plant
was shaken. I repeated these observations on some other species of Oxalis
with the same result; in some of them the movement was distinct, but was
best seen in the young leaves; in others it was extremely slight. It is a more
important fact that according to the high authority of Hofmeister, the
young shoots and leaves of all plants move after being shaken; and with
climbing plants it is, as we know, only during the early stages of growth
that the foot-stalks and tendrils are sensitive.

It is scarcely possible that the above slight movements, due to a touch
or shake in the young and growing organs of plants, can be of any func-
tional importance to them. But plants possess, in obedience to various
stimuli, powers of movement, which are of manifest importance to them;
for instance, toward and more rarely from the light — in opposition to, and
more rarely in the direction of, the attraction of gravity. When the nerves
and muscles of an animal are excited by galvanism or by the absorption


of strychnine, the consequent movements may be called an incidental re-
sult, for the nerves and muscles have not been rendered specially sensitive
to these stimuli. So with plants it appears that, from having the power of
movement in obedience to certain stimuli, they are excited in an incidental
manner by a touch or by being shaken. Hence there is no great difficulty
in admitting that in the case of leaf-climbers and tendril-bearers, it is this
tendency which has been taken advantage of and increased through natural
selection. It is, however, probable, from reasons which I have assigned in
my memoir, that this will have occurred only with plants which had already
acquired the power of revolving, and had thus become twiners.

I have already endeavored to explain how plants became twiners, namely,
by the increase of a tendency to slight and irregular revolving movements,
which were at first of no use to them; this movement, as well as that due
to a touch or shake, being the incidental result of the power of moving,
gained for other and beneficial purposes. Whether, during the gradual de-
velopment of climbing plants, natural selection has been aided by the
inherited effects of use, I will not pretend to decide; but we know that cer-
tain periodical movements, for instance the so-called sleep of plants, are
governed by habit.

I have now considered enough, perhaps more than enough, of the cases,
selected with care by a skilful naturalist to prove that natural selection is
incompetent to account for the incipient stages of useful structures; and I
have shown, as I hope, that there is no great difficulty on this head. A
good opportunity has thus been afforded for enlarging a little on gradations
of structure, often associated with strange functions — an important subject,
which was not treated at sufficient length in the former editions of this
work. I will now briefly recapitulate the foregoing cases.

With the giraffe, the continued preservation of the individuals of some
extinct high-reaching ruminant, which had the longest necks, legs, etc.,
and could browse a little above the average height, and the continued
destruction of those which could not browse so high, would have sufficed
for the production of this remarkable quadruped; but the prolonged use
of all the parts, together with inheritance, will have aided in an important
manner in their co-ordination. With the many insects which imitate various
objects, there is no improbability in the belief that an accidental resem-
blance to some common object was in each case the foundation for the
work of natural selection, since perfected through the occasional preserva-
tion of slight variations which made the resemblance at all closer; and
this will have been carried on as long as the insect continued to vary, and
as long as a more and more perfect resemblance led to its escape from
sharp-sighted enemies. In certain species of whales there is a tendency to
the formation of irregular little points of horn on the palate; and it seems
to be quite within the scope of natural selection to preserve all favorable
variations, until the points were converted, first into lamellated knobs or
teeth like those on the beak of a goose — then into short lamellae, like those


of the domestic ducks — and then into lamellae as perfect as those of the
shoveller-duck — and finally into the gigantic plates of baleen, as in the
mouth of the Greenland whale. In the family of the ducks, the lamellae are
first used as teeth, then partly as teeth and partly as a sifting apparatus, and
at last almost exclusively for this latter purpose.

With such structures as the above lamellae of horn or whalebone, habit
or use can have done little or nothing, as far as we can judge, toward their
development. On the other hand, the transportal of the lower eye of a flat-
fish to the upper side of the head, and the formation of a prehensile tail,
may be attributed almost wholly to continued use, together with inherit-
ance. With respect to the mammae of the higher animals, the most probable
conjecture is that primordially the cutaneous glands over the whole surface
of a marsupial sack secreted a nutritious fluid; and that these glands were
improved in function through natural selection, and concentrated into a
confined area, in which case they would have formed a mamma. There is
no more difficulty in understanding how the branched spines of some
ancient Echinoderm, which served as a defence, became developed through
natural selection into tridactyle pedicellariae, than in understanding the
development of the pincers of crustaceans through slight, serviceable
modifications in the ultimate and penultimate segments of a limb which
was first used solely for locomotion. In the avicularia and vibracula of the
Polyzoa we have organs widely different in appearance developed from
the same source; and with the vibracula we can understand how the suc-
cessive gradations might have been of service. With the pollinia of orchids,
the threads which originally served to tie together the pollen grains can be
traced cohering into caudicles; and the steps can likewise be followed by
which viscid matter, such as that secreted by the stigmas of ordinary flow-
ers, and still subserving nearly but not quite the same purpose, became at-
tached to the free ends of the caudicles — all these gradations being of
manifest benefit to the plants in question. With respect to climbing plants,
I need not repeat what has been so lately said.

It has often been asked, if natural selection be so potent, why has not
this or that structure been gained by certain species, to which it would
apparently have been advantageous? But it is unreasonable to expect a
precise answer to such questions, considering our ignorance of the past
history of each species, and of the conditions which at the present day
determine its numbers and range. In most cases only general reasons, but
in some few cases special reasons, can be assigned. Thus, to adapt a species
to new habits of life, many co-ordinated modifications are almost indis-
pensable, and it may often have happened that the requisite parts did not
vary in the right manner or to the right degree. Many species must have
been prevented from increasing in numbers through destructive agencies,
which stood in no relation to certain structures, which we imagine would
have been gained through natural selection from appearing to us advan-
tageous to the species. .In this case, as the struggle for life did not depend
on such structures, they could not have been acquired through natural


selection. In many cases complex and long-enduring conditions, often of a
peculiar nature, are necessary for the development of a structure; and the
requisite conditions may seldom have concurred. The belief that any given
structure, which we think, often erroneously, would have been beneficial
to a species, would have been gained under all circumstances through
natural selection, is opposed to what we can understand of its manner of
action. Mr. Mivart does not deny that natural selection has effected some-
thing; but he considers it as "demonstrably insufficient" to account for the
phenomena which I explain by its agency. His chief arguments have now
been considered, and the others will hereafter be considered. They seem to
me to partake little of the character of demonstration, and to have little
weight in comparison with those in favor of the power of natural selection,
aided by the other agencies often specified. I am bound to add, that some
of the facts and arguments here used by me, have been advanced for the
same purpose in an able article lately published in the "Medico-Chirurgical

At the present day almost all naturalists admit evolution under some
form. Mr. Mivart believes that species change through "an internal force
or tendency," about which it is not pretended that anything is known. That
species have a capacity for change, will be admitted by all evolutionists;
but there is no need, as it seems to me, to invoke any internal force beyond
the tendency to ordinary variability, which through the aid of selection by
man has given rise to many well-adapted domestic races, and which,
through the aid of natural selection, would equally well give rise by
graduated steps to natural races or species. The final result will generally
have been, as already explained, an advance, but in some few cases a
retrogression, in organization.

Mr. Mivart is further inclined to believe, and some naturalists agree with
him, that new species manifest themselves "with suddenness and by modi-
fications appearing at once." For instance, he supposes that the differences
between the extinct three-toed Hipparion and the horse arose suddenly.
He thinks it difficult to believe that the wing of a bird "was developed in
any other way than by a comparatively sudden modification of a marked
and important kind"; and apparently he would extend the same view to
the wings of bats and pterodactyles. This conclusion, which implies great
breaks or discontinuity in the series, appears to me improbable in the
highest degree.

Every one who believes in slow and gradual evolution, will of course
admit that specific changes may have been as abrupt and as great as any
single variation which we meet with under nature, or even under domestica-
tion. But as species are more variable when domesticated or cultivated than
under their natural conditions, it is not probable that such great and abrupt
variations have often occurred under nature, as are known occasionally to
rise under domestication. Of these latter variations several may be attributed
to reversion; and the characters which thus reappear were, it is probable,
in many cases at first gained in a gradual manner. A still greater number


must be called monstrosities, such as six-fingered men, porcupine men,
Ancon sheep, Niata cattle, etc.; and as they are widely different in charac-
ter from natural species, they throw very little light on our subject. Ex-
cluding such cases of abrupt variations, the few which remain would at
best constitute, if found in a state of nature, doubtful species, closely related
to their parental types.

My reasons for doubting whether natural species have changed as
abruptly as have occasionally domestic races, and for entirely disbelieving
that they have changed in the wonderful manner indicated by Mr. Mivart,
are as follows: According to our experience, abrupt and strongly marked
variations occur in our domesticated productions, singly and at rather long
intervals of time. If such occurred under nature, they would be liable, as
formerly explained, to be lost by accidental causes of destruction and by
subsequent intercrossing; and so it is known to be under domestication,
unless abrupt variations of this kind are specially preserved and separated
by the care of man. Hence, in order that a new species should suddenly
appear in the manner supposed by Mr. Mivart, it is almost necessary to
believe, in opposition to all analogy, that several wonderfully changed
individuals appeared simultaneously within the same district. This diffi-
culty, as in the case of unconscious selection by man, is avoided on the
theory of gradual evolution, through the preservation of a large number of
individuals, which varied more or less in any favorable direction, and of
the destruction of a large number which varied in an opposite manner. ^

That many species have been evolved in an extremely gradual manner,
there can hardly be a doubt. The species and even the genera of many
large natural families are so closely allied together that it is difficult to
distinguish not a few of them. On every continent, in proceeding from
north to south, from lowland to upland, etc., we meet with a host of closely
related or representative species; as we likewise do on certain distinct
continents, which we have reason to beHeve were formerly connected. But
in making these and the following remarks, I am compelled to allude to
subjects hereafter to be discussed. Look at the many outlying islands round
a continent, and see how many of their inhabitants can be raised only to
the rank of doubtful species. So it is if we look to past times, and com-
pare the species which have just passed away with those still living within
the same area; or if we compare the fossil species embedded in the sub-
stages of the same geological formation. It is indeed manifest that multi-
tudes of species are related in the closest manner to other species that still
exist, or have lately existed; and it will hardly be maintained that such
species have been developed in an abrupt or sudden manner. Nor should
it be forgotten, when we look to the special parts of allied species, instead
of to distinct species, that numerous and wonderfully fine gradations can be
traced, connecting together widely different structures.

Many large groups of facts are intelligible only on the principle that
species have been evolved by very small steps. For instance, the fact that
the species included in the larger genera are more closely related to each


other, and present a greater number of varieties, than do the species in the
smaller genera. The former are also grouped in little clusters, like varieties
round species; and they present other analogies with varieties, as was
shown in our second chapter. On this same principle we can understand
how it is that specific characters are more variable than generic characters;
and how the parts which are developed in an extraordinary degree or
manner are more variable than other parts of the same species. Many
analogous facts, all pointing in the same direction, could be added.

Although very many species have almost certainly been produced by
steps not greater than those separating fine varieties; yet it may be main-
tained that some have been developed in a different and abrupt manner.
Such an admission, however, ought not to be made without strong evidence
being assigned. The vague and in some respects false analogies, as they
have been shown to be by Mr. Chauncey Wright, which have been ad-
vanced in favor of this view, such as the sudden crystallization of inorganic
substances, or the falling of a facetted spheroid from one facet to another,
hardly deserve consideration. One class of facts, however, namely, the
sudden appearance of new and distinct forms of life in our geological
formations, supports at first sight the belief in abrupt development. But
the value of this evidence depends entirely on the perfection of the geo-
logical record, in relation to periods remote in the history of the world. If
the record is as fragmentary as many geologists strenuously assert, there is
nothing strange in new forms appearing as if suddenly developed.

Unless we admit transformations as prodigious as those advocated by
Mr. Mivart, such as the sudden development of the wings of birds or bats,
or the sudden conversion of a Hipparion into a horse, hardly any light is
thrown by the belief in abrupt modifications on the deficiency of connect-
ing links in our geological formations. But against the belief in such abrupt
changes, embryology enters a strong protest. It is notorious that the wings
of birds and bats, and the legs of horses or other quadrupeds, are undistin-
guishable at an early embryonic period, and that they become differentiated
by insensibly fine steps. Embryological resemblances of all kinds can be
accounted for, as we shall hereafter see, by the progenitors of our existing
species having varied after early youth, and having transmitted their newly
acquired characters to their offspring, at a corresponding age. The embryo
is thus left almost unaffected, and serves as a record of the past condition
of the species. Hence it is that existing species during the early stages of
their development so often resemble ancient and extinct forms belonging
to the same class. On this view of the meaning of embryological resem-
blances, and indeed on any view, it is incredible that an animal should
have undergone such momentous and abrupt transformations as those
above indicated, and yet should not bear even a trace in its embryonic
condition of any sudden modification, every detail in its structure being
developed by insensibly fine steps.

He who believes that some ancient form was transformed suddenly
through an internal force or tendency into, for instance, one furnished with


wings, will be almost compelled to assume, in opposition to all analogy,
that many individuals varied simultaneously. It cannot be denied that such
abrupt and great changes of structure are widely different from those
which most species apparently have undergone. He will further be com-
pelled to believe that many structures beautifully adapted to all the other
parts of the same creature, and to the surrounding conditions, have been
suddenly produced; and of such complex and wonderful coadaptations, he
will not be able to assign a shadow of an explanation. He will be forced to
admit that these great and sudden transformations have left no trace of
their action on the embryo. To admit all this is, as it seems to me, to enter
into the realms of miracle, and to leave those of science.



Instincts Comparable with Habits, but Different in Their Origin — Instincts Gradu-
ated — Aphides and Ants — Instincts Variable — Domestic Instincts, Their Origin
— Natural Instincts of the Cuckoo, Molothrus, Ostrich and Parasitic Bees — Slave-
making Ants — Hive-Bee, its Cell-making Instinct — Changes of Instinct and
Structure not Necessarily Simultaneous — Difficulties of the Theory of the
Natural Selection of Instincts — Neuter or Sterile Insects — Summary.

Many instincts are so wonderful that their development will probably
appear to the reader a difficulty sufficient to overthrow my whole theory. I
may here premise, that I have nothing to do with the origin of the mental
powers, any more than I have with that of life itself. We are concerned
only with the diversities of instinct and of the other mental faculties in
animals of the same class.

I will not attempt any definition of instinct. It would be easy to show that
several distinct mental actions are commonly embraced by this term; but
every one understands what is meant, when it is said that instinct impels
the cuckoo to migrate and to lay her eggs in other birds' nests. An action,
which we ourselves require experience to enable us to perform, when per-
formed by an animal, more especially by a very young one, without experi-
ence, and when performed by many individuals in the same way, without
their knowing for what purpose it is performed, is usually said to be in-
stinctive. But I could show that none of these characters are universal. A
little dose of judgment or reason, as Pierre Huber expresses it, often comes
into play, even with animals low in the scale of nature.

Frederick Guvier and several of the older metaphysicians have com-
pared instinct with habit. This comparison gives, I think, an accurate
notion of the frame of mind under which an instinctive action is per-
formed, but not necessarily of its origin. How unconsciously many habitual
actions are performed, indeed not rarely in direct opposition to our con-
scious will! yet they may be modified by the will or reason. Habits easily
become associate with other habits, with certain periods of time and states
of the body. When once acquired, they often remain constant throughout
life. Several other points of resemblance between instincts and habits could
be pointed out. As in repeating a well-known song, so in instincts, one
action follows another by a sort of rhythm; if a person be interrupted in a
song, or in repeating anything by rote, he is generally forced to go back to
recover the habitual train of thought; so P. Huber found it was with a
caterpillar, which makes a very complicated hammock; for if he took a
caterpillar which had completed its hammock up to, say, the sixth stage of
construction, and put it into a hammock completed up only to the third
stage, the caterpillar simply re-performed the fourth, fifth and sixth stages
of construction. If, however, a caterpillar were taken out of a hammock
made up, for instance, to the third stage, and were put into one finished



up to the sixth stage^ so that much of its work was already done for it, far
from deriving any benefit from this, it was much embarrassed, and in order
to complete its hammock, seemed forced to start from the third stage, where
it had left off, and thus tried to complete the already finished work.

If we suppose any habitual action to become inherited — and it can be
shown that this does sometimes happen — then the resemblance between
what originally was a habit and an instinct becomes so close as not to be
distinguished. If Mozart, instead of playing the piano-forte at three years
old with wonderfully little practice, had played a tune with no practice at
all, he might truly be said to have done so instinctively. But it would be a
serious error to suppose that the greater number of instincts have been
acquired by habit in one generation, and then transmitted by inheritance
to succeeding generations. It can be clearly shown that the most wonderful
instincts with which we are acquainted, namely, those of the hive-bee and
of many ants, could not possibly have been acquired by habit.

It will be universally admitted that instincts are as important as cor-
poreal structures for the welfare of each species, under its present condi-
tions of life. Under changed conditions of life, it is at least possible that
slight modifications of instinct might be profitable to a species; and if
it can be shown that instincts do vary ever so little, then I can see no
difficulty in natural selection preserving and continually accumulating vari-
ations of instinct to any extent that was profitable. It is thus, as I believe,
that all the most complex and wonderful instincts have originated. As modi-
fications of corporeal structure arise from, and are increased by, use or
habit, and are diminished or lost by disuse, so I do not doubt it has been
with instincts. But I believe that the effects of habit are in many cases of
subordinate importance to the effects of the natural selection of what may

Online LibraryCharles DarwinThe origin of species → online text (page 23 of 50)