Descent of Man [ 1871]

The spinal cord in man extends only as far downwards as the last
dorsal or first lumbar vertebra; but a thread-like structure (the
filum terminale) runs down the axis of the sacral part of the spinal
canal, and even along the back of the coccygeal bones. The upper
part of this filament, as Prof. Turner informs me, is undoubtedly
homologous with the spinal cord; but the lower part apparently
consists merely of the pia mater, or vascular investing membrane. Even
in this case the os coccyx may be said to possess a vestige of so
important a structure as the spinal cord, though no longer enclosed
within a bony canal. The following fact, for which I am also
indebted to Prof. Turner, shews how closely the os coccyx
corresponds with the true tail in the lower animals: Luschka has
recently discovered at the extremity of the coccygeal bones a very
peculiar convoluted body, which is continuous with the middle sacral
artery; and this discovery led Krause and Meyer to examine the tail of
a monkey (Maeacus), and of a cat, in both of which they found a
similarly convoluted body, though not at the extremity.
  The reproductive system offers various rudimentary structures; but
these differ in one important respect from the foregoing cases. Here
we are not concerned with the vestige of a part which does not
belong to the species in an efficient state, but with a part efficient
in the one sex, and represented in the other by a mere rudiment.
Nevertheless, the occurrence of such rudiments is as difficult to
explain, on the belief of the separate creation of each species, as in
the foregoing cases. Hereafter I shall have to recur to these
rudiments, and shall shew that their presence generally depends merely
on inheritance, that is, on parts acquired by one sex having been
partially transmitted to the other. I will in this place only give
some instances of such rudiments. It is well known that in the males
of all mammals, including man, rudimentary mammae exist. These in
several instances have become well developed, and have yielded a
copious supply of milk. Their essential identity in the two sexes is
likewise shewn by their occasional sympathetic enlargement in both
during an attack of the measles. The vesicula prostatica, which has
been observed in many male mammals, is now universally acknowledged to
be the homologue of the female uterus, together with the connected
passage. It is impossible to read Leuckart's able description of
this organ, and his reasoning, without admitting the justness of his
conclusion. This is especially clear in the case of those mammals in
which the true female uterus bifurcates, for in the males of these the
vesicula likewise bifurcates.* Some other rudimentary structures
belonging to the reproductive system might have been here adduced.*(2)

  * Leuckart, in Todd's Cyclopaedia of Anatomy, 1849-52, vol. iv.,
p. 1415. In man this organ is only from three to six lines in
length, but, like so many other rudimentary parts, it is variable in
development as well as in other characters.
  *(2) See, on this subject, Owen, Anatomy of Vertebrates, vol.
iii., pp. 675, 676, 706.

  The bearing of the three great classes of facts now given is
unmistakeable. But it would be superfluous fully to recapitulate the
line of argument given in detail in my Origin of Species. The
homological construction of the whole frame in the members of the same
class is intelligible, if we admit their descent from a common
progenitor, together with their subsequent adaptation to diversified
conditions. On any other view, the similarity of pattern between the
hand of a man or monkey, the foot of a horse, the flipper of a seal,
the wing of a bat, &c., is utterly inexplicable.* It is no
scientific explanation to assert that they have all been formed on the
same ideal plan. With respect to development, we can clearly
understand, on the principle of variation supervening at a rather late
embryonic period, and being inherited at a corresponding period, how
it is that the embryos of wonderfully different forms should still
retain, more or less perfectly, the structure of their common
progenitor. No other explanation has ever been given of the marvellous
fact that the embryos of a man, dog, seal, bat, reptile, &c., can at
first hardly be distinguished from each other. In order to
understand the existence of rudimentary organs, we have only to
suppose that a former progenitor possessed the parts in question in
a perfect state, and that under changed habits of life they became
greatly reduced, either from simple disuse, or through the natural
selection of those individuals which were least encumbered with a
superfluous part, aided by the other means previously indicated.

  * Prof. Bianconi, in a recently published work, illustrated by
admirable engravings (La Theorie Darwinienne et la creation dite
independante, 1874), endeavours to show that homological structures,
in the above and other cases, can be fully explained on mechanical
principles, in accordance with their uses. No one has shewn so well,
how admirably such structures are adapted for their final purpose; and
this adaptation can, as I believe, be explained through natural
selection. In considering the wing of a bat, he brings forward (p.
218) what appears to me (to use Auguste Comte's words) a mere
metaphysical principle, namely, the preservation "in its integrity
of the mammalian nature of the animal." In only a few cases does he
discuss rudiments, and then only those parts which are partially
rudimentary, such as the little hoofs of the pig and ox, which do
not touch the ground; these he shows clearly to be of service to the
animal. It is unfortunate that he did not consider such cases as the
minute teeth, which never cut through the jaw in the ox, or the mammae
of male quadrupeds, or the wings of certain beetles, existing under
the soldered wing-covers, or the vestiges of the pistil and stamens in
various flowers, and many other such cases. Although I greatly
admire Prof. Bianconi's work, yet the belief now held by most
naturalists seems to me left unshaken, that homological structures are
inexplicable on the principle of mere adaptation.