the reproductive system might have been here adduced. (55. 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, etc., is utterly inexplicable. (56. Prof. Bianconi,
in a recently published work, illustrated by admirable engravings ( La
Theorie Darwinienne et la creation dite independante, 1874),
endeavours to shew 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 shews
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.) 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 variations 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
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