PEOPLING OF LAND AND SEA 153 



carrying at their extremity a shell hardly as large as a rose- 

 hip, and held by terminal appendages transformed into 

 hooks. 



Certain species * have as allies epizoic Ccelenterates 

 which live upon their shell. When the Crustacean and 

 the Ccelenterate are young everything happens in the 

 same manner as in the case of the littoral hermit-crab ; 

 but the hermit does not change his shell as he grows, 

 whereas the polyp can produce, by the budding process, 

 other polyps similar to itself. The young family soon gets 

 too large for the shell, and spreads directly over the un- 

 sheltered portion of the hermit-crab's body, which thus 

 finds itself protected by a living cloak which always fits it. 

 This living cloak adapts itself so closely to the Crustacean that 

 it always retains the same shape, and it would be difficult 

 indeed to insist, in this case, that the form is not the direct 

 result of the conditions of development imposed upon the 

 relatively passive family of the polyp by the action of the 

 hermit-crab. Here we have a clear case of the influence of 

 external circumstances in the determination of organic forms. 



The fact that the deep sea hermit-crabs can do without a pro- 

 tective shell Ostraconotus, or content themselves with an illusory 

 one, implies that they do not run any great dangers and that 

 consequently the struggle for existence is not, in this region, 

 very intense. Indeed, whatever be the group under con- 

 sideration, the number of individuals found is apparently 

 too small to lead to serious competition. This dissociation of 

 species is not due to natural selection. It is possible that 

 species of the same genus which are distinct in the abyssal 

 fauna are descended from species which were already distinct, 

 although belonging to the same genus in the littoral region. 

 It is also possible, even if we adopt the rarely applicable test 

 of their inability to unite among themselves, that their 

 formation may be a question of the chemical conditions 

 surrounding them, and that therefore the normal species may 

 be formed in this way by dissociation of the same type at 

 any particular depth. The chemical view-point is the best one 

 we can adopt if we wish to explain the perfectly useless colour 

 of animals living in complete obscurity. 



1 Pagurus pilimanus. 



