138 READINGS IN EVOLUTION, GENETICS, AND EUGENIC S 



of descent with modification that the animal or plant must have been 

 subject to the modifying influences for an enormously long series of 

 generations. And this combined testimony of a number of organs in 

 the same organism is what the theory of descent would lead us to 

 expect, while the rival theory of design can offer no explanation of the 

 fact, that when one organ shows a conspicuous departure from the 

 supposed ideal type, some of the other organs in the same organism 

 should tend to keep it company by doing likewise. 



As an illustration both of this and of other points which have been 

 mentioned, I may draw attention to what seems to me a particularly 

 suggestive case. So-called soldier- or hermit-crabs are crabs which 

 have adopted the habit of appropriating the empty shells of moUusks. 

 In association with this peculiar habit, the structure of these animals 

 differs very greatly from that of all other crabs. In particular, the 

 hinder part of the body, which occupies the mollusk-shell, and which 

 therefore has ceased to require any hard covering of its own, has been 

 suffered to lose its calcareous integument, and presents a soft fleshy 

 character, quite unlike that of the most exposed parts of the animal. 

 Moreover, this soft fleshy part of the creature is especially adapted to 

 the particular requirements of the creature by having its lateral 

 appendages — i.e., appendages which in other Crustacea perform the 

 function of legs — modified so as to act as claspers to the inside of the 

 mollusk-shell; while the tail-end of the part in question is twisted 

 into the form of a spiral, which fits into the spiral of the mollusk-shell. 

 Now, in Keeling Island there is a large kind of crab called Birgus latro, 

 which lives upon land and there feeds upon cocoa-nuts. The whole 

 structure of this crab, it seems to me, unmistakably resembles the 

 structure of a hermit-crab (Fig. 16). 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, 

 I think we may infer that the ancestors of Birgus were hermit-crabs 

 living in mollusk-shells; but that their descendants gradually relin- 

 quished 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 a 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 



