104 THE EVOLUTION THEORY 



part. At this particular spot, then, the chitin will be specially 

 thick. 



This clearly implies that each particular part of the skin has its 

 specific nutritive substances, necessitated by the nerves which traverse 

 it ! Thus there must be as many nerve-nutritive substances as there 

 are skin-nerves, sj^ecific chemical combinations for every part of the 

 body which is capable of heritable variation. This is so extra- 

 ordinarily improbable that I need say nothing more about it. If 

 the Lamarckian principle requires this kind of hypothesis to bolster 

 it up, it is undoubtedly doomed. 



If we disregard altogether the positive aspect of Zehnder's 

 hypothesis, and assume that the skin-nerves are really stimulated 

 through the chitin by every strain and pressure to which a spot 

 of skin is exposed, and that they cause a correspondingly greater 

 secretion of chitin, which would then, according to the Lamarckian 

 principle, be hereditary, does this harmonize with what actually 

 occurs in the development of the skeleton as we know it in the 

 case of Insects and Crustaceans ? Not at all ! Can we suppose that 

 the carapace of a crab or the enormously hard wing-covers of a water- 

 beetle are exposed to a continual pounding and pressing and pushing ? 

 Exactly the contrary is the case. Every assailant takes care not to 

 grasp the animal where it is so well protected, and seeks out the 

 most vulnerable parts for its attacks. It may be answered that, 

 while this is certainly the case now, the animals were badly protected 

 when the ancestral forms were evolving. But that they could not 

 have become hard by dint of being frequently bitten or otherwise 

 wounded should be obvious from the fact that the whole of the 

 wing-covers and the whole of the carapace is uniformly covered 

 with thick chitin, while each wound would only stimulate particular 

 spots ; and we should also have to admit that, since these parts of 

 the skin which are now so well protected are no longer seized and 

 stimulated, they would long ago have become thin again, according 

 to the principle of the degeneration of parts no longer used, or, in 

 this case, no longer stimulated. But there is no need for wastino- 

 time over such quibbles, since there is a fact which absolutely contra- 

 dicts Zehnder's hypothesis. I mean the degeneration of the chitinous 

 skeleton in those Crustaceans and Insects which protect the abdomen 

 within a shelter like the hermit-crabs, the caddis- flies (Phryganidse), 

 (Fig. 107) and the sack-carrying caterpillars of the Psychidas among 

 Lepidoptera. The hermit-crabs, as is well known, squeeze their 

 abdomen into a usually spirally-coiled Gasteropod shell, and they 

 always choose houses which are wide enough to conceal the whole 



