I40 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 ppint 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 alUed 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 



