THE ARGUMENTS FROM MORPHOLOGY.
§ 133. Leaving out of consideration those parallelisms among their modes of
development which characterize organisms belonging to each group, that
community of plan which exists among them when mature is extremely
remarkable and extremely suggestive. As before shown (§ 103), neither the
supposition that these combinations of attributes which unite classes are
fortuitous, nor the supposition that no other combinations were
practicable, nor the supposition of adherence to pre-determined typical
plans, suffices to explain the facts. An instance will best prepare the
reader for seeing the true meaning of these fundamental likenesses.
Under the immensely-varied forms of insects, greatly elongated like the
dragon-fly or contracted in shape like the lady-bird, winged like the
butterfly or wingless like the flea, we find this character in
common--there are primarily seventeen segments.[49] These segments may be
distinctly marked or they may be so fused as to make it difficult to find
the divisions between them, but they always exist. What now can be the
meaning of this community of structure throughout the hundred thousand
kinds of insects filling the air, burrowing in the earth, swimming in the
water? Why under the down-covered body of a moth and under the hard
wing-cases of a beetle, should there be discovered the same number of
divisions? Why should there be no more somites in the Stick-insect, or
other Phasmid a foot long, than there are in a small creature like the
louse? Why should the inert _Aphis_ and the swift-flying Emperor-butterfly
be constructed on the same fundamental plan? It cannot be by chance that
there exist equal numbers of segments in all these multitudes of species.
There is no reason to think it was _necessary_, in the sense that no other
number would have made a possible organism. And to say that it is the
result of _design_--to say that the Creator followed this pattern
throughout, merely for the purpose of maintaining the pattern--is to assign
an absurd motive. No rational interpretation of these and countless like
morphological facts, can be given except by the hypothesis of evolution;
and from the hypothesis of evolution they are corollaries. If organic forms
have arisen from common stocks by perpetual divergences and
re-divergences--if they have continued to inherit, more or less clearly,
the characters of ancestral races; then there will naturally result these
communities of fundamental structure among creatures which have severally
become modified in multitudinous ways and degrees, in adaptation to their
respective modes of life. To this let it be added that while the belief in
an intentional adhesion to a pre-determined pattern throughout a whole
group, is negatived by the occurrence of occasional deviations from the
pattern; such deviations are reconcilable with the belief in evolution. As
pointed out in the last chapter, ancestral traits will be obscured more or
less according as the superposed modifications of structure, have or have
not been furthered by the conditions of life and development to which the
type has been subjected.
§ 134. Besides these wide-embracing and often deeply-hidden homologies,
which hold together different animals, there are the scarcely-less
significant homologies between different organs of the same animal. These,
like the others, are obstacles to the supernatural interpretations and
supports of the natural interpretation.
One of the most familiar and instructive examples is furnished by the
vertebral column. Snakes, which move sinuously through and over plants and
stones, obviously need a segmentation of the bony axis from end to end; and
inasmuch as flexibility is required throughout the whole length of the
body, there is advantage in the comparative uniformity of this
segmentation. The movements would be impeded if, instead of a chain of
vertebræ varying but little in their lengths, there existed in the middle
of the series some long bony mass that would not bend. But in the higher
_Vertebrata_, the mechanical actions and reactions demand that while some
parts of the vertebral column shall be flexible, other parts shall be
inflexible. Inflexibility is specially requisite in that part of it called
the sacrum; which, in mammals and birds, forms a fulcrum exposed to the
greatest strains the skeleton has to bear. Now in both mammals and birds,
this rigid portion of the vertebral column is not made of one long segment
or vertebra, but of several segments fused together. In man there are five
of these confluent sacral vertebræ; and in the ostrich tribe they number
from seventeen to twenty. Why is this? Why, if the skeleton of each species
was separately contrived, was this bony mass made by soldering together a
number of vertebræ like those forming the rest of the column, instead of
being made out of one single piece? And why, if typical uniformity was to
be maintained, does the number of sacral vertebræ vary within the same
order of birds? Why, too, should the development of the sacrum be by the
round-about process of first forming its separate constituent vertebræ, and
then destroying their separateness? In the embryo of a mammal or bird, the
central element of the vertebral column is, at the outset, continuous. The
segments that are to become vertebræ, arise gradually in the adjacent
mesoderm, and enwrap this originally-homogeneous axis or notochord. Equally
in those parts of the spine which are to remain flexible, and in those
parts which are to grow rigid, these segments are formed; and that part of
the spine which is to compose the sacrum, having acquired this segmental
structure, loses it again by coalescence of the segments. To what end is
this construction and re-construction? If, originally, the spine in
vertebrate animals consisted from head to tail of separate moveable
segments, as it does still in fishes and some reptiles--if, in the
evolution of the higher _Vertebrata_, certain of these moveable segments
were rendered less moveable with respect to one another, by the mechanical
conditions they were exposed to, and at length became relatively immovable;
it is comprehensible why the sacrum formed out of them, should continue
ever after to show its originally-segmented structure. But on any other
hypothesis this segmented structure is inexplicable. "We see the same law
in comparing the wonderfully complex jaws and legs in crustaceans," says
Mr. Darwin: referring to the fact that those numerous lateral appendages
which, in the lower crustaceans, most of them serve as legs, and have like
shapes, are, in the higher crustaceans, some of them represented by
enormously-developed claws, and others by variously-modified foot-jaws. "It
is familiar to almost every one," he continues, "that in a flower the
relative position of the sepals, petals, stamens, and pistils, as well as
their intimate structure, are intelligible on the view that they consist of
metamorphosed leaves arranged in a spire. In monstrous plants we often get
direct evidence of the possibility of one organ being transformed into
another; and we can actually see in embryonic crustaceans and in many other
animals, and in flowers, that organs, which when mature become extremely
different, are at an early stage of growth exactly alike." ... "Why should
one crustacean, which has an extremely complex mouth formed of many parts
consequently always have fewer legs; or conversely, those with many legs
have simpler mouths? Why should the sepals, petals, stamens, and pistils in
any individual flower, though fitted for such widely-different purposes, be
all constructed on the same pattern?"
To these and countless similar questions, the theory of evolution furnishes
the only rational answer. In the course of that change from homogeneity to
heterogeneity of structure displayed in evolution under every form, it will
necessarily happen that from organisms made up of numerous like parts,
there will arise organisms made up of parts more and more unlike: which
unlike parts will nevertheless continue to bear traces of their primitive
likeness.
§ 135. One more striking morphological fact, near akin to some of the facts
dwelt on in the last chapter, must be here set down--the frequent
occurrence, in adult animals and plants, of rudimentary and useless organs,
which are homologous with organs that are developed and useful in allied
animals and plants. In the last chapter we saw that during the development
of embryos, there often arise organs which disappear on being replaced by
other organs discharging the same functions in better ways; and that in
some cases, organs develop to certain points and are then re-absorbed
without performing any functions. Very generally, however, the
partially-developed organs are retained throughout life.
The osteology of the higher _Vertebrata_ supplies abundant examples.
Vertebral processes which, in one tribe, are fully formed and ossified from
independent centres, are, in other tribes, mere tubercles not having
independent centres of ossification. While in the tail of this animal the
vertebræ are severally composed of centrum and appendages, in the tail of
that animal they are simple osseous masses without any appendages; and in
another animal they have lost their individualities by coalescence with
neighbouring vertebræ into a rudimentary tail. From the structures of the
limbs analogous facts are cited by comparative anatomists. The undeveloped
state of certain metacarpal bones, characterizes whole groups of mammals.
In one case we find the normal number of digits; and, in another case, a
smaller number with an atrophied digit to make out the complement. Here is
a digit with its full number of phalanges; and there a digit of which one
phalange has been arrested in its growth. Still more remarkable are the
instances of entire limbs being rudimentary; as in certain snakes, which
have hind legs hidden beneath the integument. So, too, is it with dermal
appendages. Some of the smooth-skinned amphibia have scales buried in the
skin. The seal, which is a mammal considerably modified in adaptation to an
aquatic life, and which uses its feet mainly as paddles, has toes that
still bear external nails; but the manatee, which is a much more
transformed mammal, has nailless paddles which, when the skin is removed,
are said, by Humboldt, to display rudimentary nails at the ends of the
imbedded digits. Nearly all birds are covered with developed feathers,
severally composed of a shaft bearing fibres, each of which, again, bears a
fringe of down. But in some birds, as in the ostrich, various stages of
arrested development of the feathers may be traced: between the
unusually-elaborated feathers of the tail, and those about the beak which
are reduced to simple hairs, there are transitions. Nor is this the extreme
case. In the _Apteryx_ we see the whole of the feathers reduced to a
hair-like form. Again, the hair which commonly covers the body in mammals
is, over the greater part of the human body almost rudimentary, and is in
some parts reduced to mere down--down which nevertheless proves itself to
be homologous with the hair of mammals in general, by occasionally
developing into the original form. Numerous cases of aborted organs are
given by Mr. Darwin, of which a few may be here added. "Nothing can be
plainer," he remarks, "than that wings are formed for flight, yet in how
many insects do we see wings so reduced in size as to be utterly incapable
of flight, and not rarely lying under wing-cases, firmly soldered
together?" ... "In plants with separated sexes, the male flowers often have
a rudiment of a pistil; and Kölreuter found that by crossing such male
plants with an hermaphrodite species, the rudiment of the pistil in the
hybrid offspring was much increased in size; and this shows that the
rudiment and the perfect pistil are essentially alike in nature." And then,
to complete the proof that these undeveloped parts are marks of descent
from races in which they were developed, there are not a few direct
experiences of this relation. "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 re-appearance of
minute dangling horns in hornless breeds of cattle." (_Origin of Species_,
1859, pp. 451, 454.)
Here, as before, the teleological doctrine fails utterly; for these
rudimentary organs are useless, and occasionally even detrimental; as is
the _appendix vermiformis_, in Man--a part of the cæcum which is of no
value for the purpose of absorption but which, by detaining small foreign
bodies, often causes severe inflammation and death. The doctrine of typical
plans is equally out of court; for while, in some members of a group,
rudimentary organs completing the general type are traceable, in other
members of the same group such organs are unrepresented. There remains only
the doctrine of evolution; and to this, these rudimentary organs offer no
difficulties. On the contrary, they are among its most striking evidences.
§ 136. The general truths of morphology thus coincide in their
implications. Unity of type, maintained under extreme dissimilarities of
form and mode of life, is explicable as resulting from descent with
modification; but is otherwise inexplicable. The likenesses disguised by
unlikenesses, which the comparative anatomist discovers between various
organs in the same organism, are worse than meaningless if it be supposed
that organisms were severally framed as we now see them; but they fit in
quite harmoniously with the belief that each kind of organism is a product
of accumulated modifications upon modifications. And the presence, in all
kinds of animals and plants, of functionally-useless parts corresponding to
parts that are functionally-useful in allied animals and plants, while it
is totally incongruous with the belief in a construction of each organism
by miraculous interposition, is just what we are led to expect by the
belief that organisms have arisen by progression.