I 5 8 KEGENERA TION 



tation, but the crab has the same mechanism for the slender walking 

 legs, that one would suppose could be easily withdrawn from the old 

 covering. It should also be remembered that we do not know 

 whether the breaking-joint at the base of the leg of the crab and 

 of the crayfish would act at the time when the leg is being with- 

 drawn from the old exoskeleton, unless the leg were first injured 

 outside of the joint. 



Our analysis leads to the conclusion that we can neither account 

 for the phenomenon of autotomy as due to internal causes alone in 

 the sense of its being a general property of protoplasm, nor to an 

 external cause, in the sense of a reaction to injury or loss from 

 accident. There would seem then only one possibility left, namely, 

 that it is a result of both together, or in other words, a process that 

 the animal has acquired in connection with the conditions under 

 which it lives, or in other words, an adaptive response of the organism 

 to its conditions of life. 



We are not, however, able at present to push these questions 

 farther, for, however probable it may seem that animals and plants 

 may acquire characteristics useful to them in their special conditions 

 of life, and yet not of sufficient importance to be decisive in a life and 

 death struggle, still we cannot, at present, state how this could have 

 taken place in the course of evolution. For, however plausible it 

 may appear that the useful structure has been built up through an 

 interaction between the organism and its environment, we cannot 

 afford to leave out of sight another possibility, viz. that the struc- 

 ture or action may have appeared independently of the environ- 

 ment, but after it appeared the organism adopted a new environment 

 to which its new characters made it better suited. If the latter alter- 

 native is true, we should look in vain if we tried to find out how the 

 interaction of the environment brought about the adaptation. The 

 relation would not be a causal one, in a physical sense, but the out- 

 come of a different sort of a relation, viz. the restriction of the organ- 

 ism to the environment in which it can remain in existence and leave 

 descendants. 



