Morphology. 63 



Birgus /afro, which lives upon land and there feeds 

 upon cocoa-nuts. The whole structure of this crab, it 

 seems to me, unmistakeably resembles the structure 

 of a hermit-crab (see drawings on the next page, 

 Fig. 7). Yet this crab neither lives in the shell of 

 a mollusk, nor is the hinder part of its body in the soft 

 and fleshy condition just described : on the contrary, it 

 is covered with a hard integument like all the other 

 parts of the animal. Consequently,! think we may infer 

 that the ancestors of Birgus were hermit-crabs living 

 in mollusk-shells ; but that their descendants grad- 

 ually relinquished this habit as they gradually became 

 more and more terrestrial, while, concurrently with 

 these changes in habit, the originally soft posterior 

 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 ne- 

 cessity 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 



