CHAPTER IX 



ADAPTATION 



It is usual to proceed on the assumption that, if all evolu- 

 tionary divergence were adaptive, the importance of Natural 

 Selection would be finally demonstrated. We wish now to 

 examine what we know of adaptation, to see if it supports the 

 view that selection by the environment has led to adaptation 

 to it. The term adaptation, itself, is applied to several 

 phenomena which are not actually of the same nature, and we 

 must attempt to explain this difference in the use of the term. 

 Useful Characters.— Many observations have been made 

 tending to show that various structures, often apparently 

 trivial or valueless, have really some function in an animal's 

 life-history. Structures the functions of which are known or 

 have been surmised are usually described as adaptive, but, as 

 Bateson (1894, p. 12) points out, such a description is mis- 

 leading, for it is scarcely ever known in any particular case 

 whether actually the structure on the whole confers an ad- 

 vantage on the individual possessing it. One might distin- 

 guish animal structure into three categories : {a) apparently- 

 useless structures ; (b) useful structures ; and (c) adaptive 

 structures, which are not merely useful at one stage in 

 the life-history but actually confer a definite advantage not 

 counterbalanced in other ways. The distinction between 

 (b) and (c) may be readily seen in the following example. 

 Many Lampyrid beetles have the power of emitting flashes of 

 light in both sexes. Repeated observations have shown the 

 value of the flashes as a means of bringing the sexes together. 

 The light-organ therefore falls at least into category (b) of 

 useful structures. But to show that it should be placed in (c) 

 it would be necessary to prove that there are no counter- 

 balancing disadvantages— ?.£. that the light did not also attract 

 enemies to a dangerous extent, or that the energy expended in 



