140 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 
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 especially 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. 
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 rudi- 
mentary structures. 
Throughout both the animal and vegetable kingdoms we con- 
stantly 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 rudi- 
mentary 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, as Hunter long ago remarked, “is constructed upon the same 
principle as in the quadruped”’; yet, as Owen says, “the outer open- 
ing and passage leading therefrom to the tympanum can rarely be 
affected by sonorous vibrations of the atmosphere, and indeed they 
are reduced, or have degenerated, to a degree which makes it difficult 
to conceive how such vibrations can be propagated to the ear-drum 
during the brief moments in which the opening may be raised above 
the water.” 
Now, rudimentary organs of this kind are of such frequent occur- 
rence, that almost every species presents one or more of them— 
usually, indeed, a considerable number. How, then, are they to be 
accounted for? Of course the theory of descent with adaptive modi- 
fication has a simple answer to supply—namely, that when, from 
