parts in the embryo, than in the adult; so that the organ at this early age
is less rudimentary, or even cannot be said to be in any degree
rudimentary. Hence, also, a rudimentary organ in the adult, is often said
to have retained its embryonic condition.
I have now given the leading facts with respect to rudimentary organs. In
reflecting on them, every one must be struck with astonishment: for the
same reasoning power which tells us plainly that most parts and organs are
exquisitely adapted for certain purposes, tells us with equal plainness
that these rudimentary or atrophied organs, are imperfect and useless. In
works on natural history rudimentary organs are generally said to have been
created 'for the sake of symmetry,' or in order 'to complete the scheme of
nature;' but this seems to me no explanation, merely a restatement of the
fact. Would it be thought sufficient to say that because planets revolve
in elliptic courses round the sun, satellites follow the same course round
the planets, for the sake of symmetry, and to complete the scheme of
nature? An eminent physiologist accounts for the presence of rudimentary
organs, by supposing that they serve to excrete matter in excess, or
injurious to the system; but can we suppose that the minute papilla, which
often represents the pistil in male flowers, and which is formed merely of
cellular tissue, can thus act? Can we suppose that the formation of
rudimentary teeth which are subsequently absorbed, can be of any service to
the rapidly growing embryonic calf by the excretion of precious phosphate
of lime? When a man's fingers have been amputated, imperfect nails
sometimes appear on the stumps: I could as soon believe that these
vestiges of nails have appeared, not from unknown laws of growth, but in
order to excrete horny matter, as that the rudimentary nails on the fin of
the manatee were formed for this purpose.
On my view of descent with modification, the origin of rudimentary organs
is simple. We have plenty of cases of rudimentary organs in our domestic
productions,--as the stump of a tail in tailless breeds,--the vestige of an
ear in earless breeds,--the reappearance of minute dangling horns in
hornless breeds of cattle, more especially, according to Youatt, in young
animals,--and the state of the whole flower in the cauliflower. We often
see rudiments of various parts in monsters. But I doubt whether any of
these cases throw light on the origin of rudimentary organs in a state of
nature, further than by showing that rudiments can be produced; for I doubt
whether species under nature ever undergo abrupt changes. I believe that
disuse has been the main agency; that it has led in successive generations
to the gradual reduction of various organs, until they have become
rudimentary,--as in the case of the eyes of animals inhabiting dark
caverns, and of the wings of birds inhabiting oceanic islands, which have
seldom been forced to take flight, and have ultimately lost the power of
flying. Again, an organ useful under certain conditions, might become
injurious under others, as with the wings of beetles living on small and
exposed islands; and in this case natural selection would continue slowly
to reduce the organ, until it was rendered harmless and rudimentary.
Any change in function, which can be effected by insensibly small steps, is
within the power of natural selection; so that an organ rendered, during
changed habits of life, useless or injurious for one purpose, might easily
be modified and used for another purpose. Or an organ might be retained
for one alone of its former functions. An organ, when rendered useless,
may well be variable, for its variations cannot be checked by natural
selection. At whatever period of life disuse or selection reduces an
organ, and this will generally be when the being has come to maturity and
to its full powers of action, the principle of inheritance at corresponding
ages will reproduce the organ in its reduced state at the same age, and
consequently will seldom affect or reduce it in the embryo. Thus we can
understand the greater relative size of rudimentary organs in the embryo,
and their lesser relative size in the adult. But if each step of the
process of reduction were to be inherited, not at the corresponding age,
but at an extremely early period of life (as we have good reason to believe
to be possible) the rudimentary part would tend to be wholly lost, and we
should have a case of complete abortion. The principle, also, of economy,
explained in a former chapter, by which the materials forming any part or
structure, if not useful to the possessor, will be saved as far as is
possible, will probably often come into play; and this will tend to cause
the entire obliteration of a rudimentary organ.
As the presence of rudimentary organs is thus due to the tendency in every
part of the organisation, which has long existed, to be inherited--we can
understand, on the genealogical view of classification, how it is that
systematists have found rudimentary parts as useful as, or even sometimes
more useful than, parts of high physiological importance. Rudimentary
organs may be compared with the letters in a word, still retained in the
spelling, but become useless in the pronunciation, but which serve as a
clue in seeking for its derivation. On the view of descent with
modification, we may conclude that the existence of organs in a
rudimentary, imperfect, and useless condition, or quite aborted, far from
presenting a strange difficulty, as they assuredly do on the ordinary
doctrine of creation, might even have been anticipated, and can be
accounted for by the laws of inheritance.
Summary. -- In this chapter I have attempted to show, that the
subordination of group to group in all organisms throughout all time; that
the nature of the relationship, by which all living and extinct beings are
united by complex, radiating, and circuitous lines of affinities into one
grand system; the rules followed and the difficulties encountered by
naturalists in their classifications; the value set upon characters, if
constant and prevalent, whether of high vital importance, or of the most
trifling importance, or, as in rudimentary organs, of no importance; the
wide opposition in value between analogical or adaptive characters, and
characters of true affinity; and other such rules;--all naturally follow on
the view of the common parentage of those forms which are considered by
naturalists as allied, together with their modification through natural
selection, with its contingencies of extinction and divergence of
character. In considering this view of classification, it should be borne
in mind that the element of descent has been universally used in ranking
together the sexes, ages, and acknowledged varieties of the same species,
however different they may be in structure. If we extend the use of this
element of descent,--the only certainly known cause of similarity in
organic beings,--we shall understand what is meant by the natural system:
it is genealogical in its attempted arrangement, with
the grades of
acquired difference marked by the terms varieties, species, genera,
families, orders, and classes.
On this same view of descent with modification, all the great facts in
Morphology become intelligible,--whether we look to the same pattern
displayed in the homologous organs, to whatever purpose applied, of the
different species of a class; or to the homologous parts constructed on the
same pattern in each individual animal and plant.
On the principle of successive slight variations, not necessarily or
generally supervening at a very early period of life, and being inherited
at a corresponding period, we can understand the great leading facts in
Embryology; namely, the resemblance in an individual embryo of the
homologous parts, which when matured will become widely different from each
other in structure and function; and the resemblance in different species
of a class of the homologous parts or organs, though fitted in the adult
members for purposes as different as possible. Larvae are active embryos,
which have become specially modified in relation to their habits of life,
through the principle of modifications being inherited at corresponding
ages. On this same principle--and bearing in mind, that when organs are
reduced in size, either from disuse or selection, it will generally be at
that period of life when the being has to provide for its own wants, and
bearing in mind how strong is the principle of inheritance--the occurrence
of rudimentary organs and their final abortion, present to us no
inexplicable difficulties; on the contrary, their presence might have been
even anticipated. The importance of embryological characters and of
rudimentary organs in classification is intelligible, on the view that an
arrangement is only so far natural as it is genealogical.
Finally, the several classes of facts which have been considered in this
chapter, seem to me to proclaim so plainly, that the innumerable species,
genera, and families of organic beings, with which this world is peopled,
have all descended, each within its own class or group, from common
parents, and have all been modified in the course of descent, that I should
without hesitation adopt this view, even if it were unsupported by other
facts or arguments.
Chapter XIV
Recapitulation and Conclusion
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.
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.
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.
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.
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.
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.
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.
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.
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