t lines of descent--namely, the insects, reptiles, birds,
and mammals. Now if in all, or indeed in any, of these four cases the
wings had been developed on the same anatomical pattern, so as not only
to present the analogical resemblance which it is necessary that they
should present in order to discharge their common function of flying,
but likewise an homologous or structural resemblance, showing that they
had been formed on the same anatomical "plan,"--if such has been the
case, I say, the theory of natural selection would certainly be
destroyed.

Now it has been alleged by competent naturalists that there are several
such cases in organic nature. We have already noticed in a previous
chapter (pp. 58, 59), that Mr. Mivart has instanced the eye of the
cuttle-fish as not only analogous to, but also homologous with, the eye
of a true fish--that is to say, the eye of a mollusk with the eye of a
vertebrate. And he has also instanced the remarkable resemblance of a
shrew to a mouse--that is, of an insectivorous mammal to a rodent--not
to mention other cases. In the chapter alluded to these instances of
homology, alleged to occur in different branches of the tree of life,
were considered with reference to the process of organic evolution as a
fact: they are now being considered with reference to the agency of
natural selection as a method. And just as in the former case it was
shown, that if any such alleged instances could be proved, the proof
would be fatal to the general theory of organic evolution by physical
causes, so in the present case, if this could be proved, it would be
equally fatal to the more special theory of natural selection. But, as
we have before seen, no single case of this kind has ever been made
out; and, therefore, not only does this supposed objection fall to the
ground, but in so doing it furnishes an additional argument in favour of
natural selection. For in the earlier chapter just alluded to I showed
that this great and general fact of our nowhere being able to find two
homologous structures in different branches of the tree of life, was the
strongest possible testimony in favour of the theory of evolution. And,
by parity of reasoning, I now adduce it as equally strong evidence of
natural selection having been the cause of _adaptive_ structures,
independently developed in all the different lines of descent. For the
alternative is between adaptations having been caused by natural
selection or by s