being here excluded by the fact
that the animals compared belong to two widely different branches of the
tree of life, how are we to explain the identity of type manifested by
these two complicated organs of vision? The only hypothesis open to us
is intelligent adherence to an ideal plan or mechanism. But as this
cannot now be urged in any comparable case throughout the whole organic
world, we may on the other hand present it as a most significant fact,
that while within the limits of the same large branch of the tree of
life we constantly find the same typical structures modified so as to
perform very different functions, we never find any of these particular
types of structure in other large branches of the tree. That is to say,
we never find typical structures appearing except in cases where their
presence may be explained by the hypothesis of hereditary descent; while
in thousands of such cases we find these structures undergoing every
conceivable variety of adaptive modification.

Consequently, special creationists must fall back upon another position
and say,--Well, but it may have pleased the Deity to form a certain
number of ideal types, and never to have allowed the structures
occurring in one type to appear in any of the others. We
answer,--Undoubtedly such may have been the case; but, if so, it is a
most unfortunate thing for your theory, because the fact implies that
the Deity has planned his types in such a way as to suggest the
counter-theory of descent. For instance, it would seem most capricious
on the part of the Deity to have made the eyes of an innumerable number
of fish on exactly the same ideal type, and then to have made the eye of
the octopus so exactly like these other eyes in superficial appearance
as to deceive so accomplished a naturalist as Mr. Mivart, and yet to
have taken scrupulous care that in no one ideal particular should the
one type resemble the other. However, adopting for the sake of argument
this great assumption, let us suppose that God did lay down these
arbitrary rules for his own guidance in creation, and then let us see to
what the assumption leads. If the Deity formed a certain number of ideal
types, and determined that on no account should he allow any part of one
type to appear in any part of another, surely we should expect that
within the limits of the same type the same typical structures should
always be present. Thus, remember what efforts, so to speak, have been
made