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 
jof 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 
