hermit-crab to an ordinary crab,
so far as living without the necessity of a mollusk-shell is concerned.
From an evolutionary point of view, therefore, we appear to have in the
existing structure of _Birgus_ a morphological record of all these
changes, and one which gives us a reasonable explanation of why the
animal presents the extraordinary appearance which it does. But, on the
theory of special creation, it is inexplicable why this land-crab should
have been formed on the pattern of a hermit-crab, when it never has need
to enter the shell of a mollusk. In other words, its peculiar
structure is not specially in keeping with its present habits, although
so curiously allied to the similar structure of certain other crabs of
totally different habits, in relation to which the peculiarities are of
plain and obvious significance.

[Illustration: FIG. 7.--Hermit-crabs compared with the cocoa-nut
crab. On the left of the illustration one hermit-crab is represented
as occupying a mollusk-shell, and another (larger specimen) as it
appears when withdrawn from such a shell. On the right of the
illustration the cocoa-nut crab is represented in its natural
habitat on land. When full-grown, however, it is much larger than
our hermit-crabs. The latter are drawn from life, natural size, the
former from a specimen in the British Museum, 1/6 natural size.]

* * * * *

I will devote the remainder of this chapter to considering another
branch of the argument from morphology, to which the case of _Birgus_
serves as a suitable introduction: I mean the argument from rudimentary
structures.

Throughout both the animal and vegetable kingdoms we constantly meet
with dwarfed and useless representatives of organs, which in other and
allied kinds of animals and plants are of large size and functional
utility. Thus, for instance, the unborn whale has rudimentary teeth,
which are never destined to cut the gums; and throughout its life this
animal retains, in a similarly rudimentary condition, a number of organs
which never could have been of use to any kind of creature save a
terrestrial quadruped. The whole anatomy of its internal ear, for
example, has reference to hearing in air--or, as Hunter long ago
remarked, "is constructed upon the same principle as in the quadruped";
yet, as Owen says, "the outer opening and passage leading therefrom to
the tympanum can rarely be affected by