62 Darwin, and after Darwin. 



position 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 mollusks. 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 cover- 

 ing of its own, has been suffered to lose its calcareous 

 integument, and presents a soft fleshy character, quite 

 unlike that of the more exposed parts of the animal. 

 Moreover, this soft fleshy part of the creature is 

 specially 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 



