364 THE VARIATION OF ANIMALS IN NATURE 



nor is it less, incomprehensible than any of the properties 

 enumerated. A living being devoid of purposive structures 

 would be inconceivable. To comprehend the origin of adapta- 

 tions in the living being is to comprehend the essence of life. 

 And the essence of life is no easier to comprehend than the 

 essence of matter, energy, feeling, consciousness and will.' 

 Without, perhaps, adopting so extreme an attitude, we may 

 still believe that the tendency to maintain form and indivi- 

 duality is a more fundamental characteristic of living organisms 

 than the tendency to change under external pressure, and we 

 are led to contrast the so-called ' internal mechanisms,' which 

 are the very life-blood of the organism, with the ' adaptations 

 to external nature,' which indicate, at least in part, where the 

 environment has induced modifications. We may further 

 compare the two types of adaptation in their relation to sur- 

 vival : the first type is so essential to the organism that life 

 would be impossible if even a small detail of the mechanism 

 were out of order ; the second type, even on the most enthu- 

 siastic view, is usually only helpful in emergencies or in some 

 small part of the life-history, and even then is not literally 

 essential to life. A similar comparison has been made by 

 D'Arcy Thompson (191 7, p. 617), who, taking an extreme 

 view, says (of the study of the second type of adaptations) : 

 { The fate of such arguments or illustrations [protective and 

 warning coloration, etc.] is always the same. They attract 

 and captivate for a while, they go to the building of a creed, 

 which contemporary orthodoxy defends under its severest 

 penalties ; but the time comes when they lose their fascina- 

 tion, they somehow cease to satisfy and to convince, their 

 foundations are discovered to be insecure, and in the end no 

 man troubles to controvert them. But of a different order 

 from all such " adaptations " as these are those very perfect 

 adaptations of form which, for instance, fit a fish for swimming 

 or a bird for flight. Here we are far above the region of mere 

 hypothesis, for we have to deal with questions of mechanical 

 efficiency where statical and dynamical considerations can be 

 applied and established in detail.' 



The passage just quoted brings us to the crucial question 

 in the problem of adaptation — the relation between the perfec- 

 tion of internal organisation and specialisation for a particular 

 mode of life. We believe that biology is at present very far 



