parts acquired a hard protective covering to take the place of that which was formerly supplied by the mollusk-shell. So that, if so, we now have, within the limits of a single organism, evidence of a whole series of morphological changes in the past history of its species. First, there must have been the great change from an ordinary crab to a hermit-crab in all the respects previously pointed out. Next, there must have been the change back again from a 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one hermit-crab is represented as occupying a mollusk-shell, and another (larger specimen) as it appears when withdrawn from such a shell.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.