ADAPTATION 351 



Selection, while evolutionary progress was maintained only by 

 lines which escaped the action of selection with the fatal, blind- 

 alley specialisation which it entails. To illustrate the argu- 

 ment by a metaphor, we may compare the evolution of a 

 species with the course of a boat down a stream. The banks 

 represent the selecting environment. If the stream is narrow 

 and the boat is undirected, then the banks will narrowly 

 determine the course pursued and the boat will eventually 

 show signs of its frequent collisions. But if the stream be 

 very broad it is easy to imagine that even a moderately well- 

 steered boat may within wide limits have a safer journey. 

 For ' adaptable ' species the stream is very broad. 



We do not wish to push this speculation any further at the 

 present stage of our discussion, but it may be noted that the 

 relation between the rate of specialisation and the rate of 

 change of the environment in any particular habitat would be 

 of importance. 



Statistical Adaptation. — A third conception of adaptation 

 may be called the statistical. 1 From this point of view the rather 

 exceptional interrelationships, such as those mentioned in the 

 previous paragraphs, are less stressed, and the greatest import- 

 ance is attributed to the highly complex environment in which 

 the species must live. If the environment is the sum of 

 a number of conflicting and highly variable influences, no 

 species can be adapted in all directions to the theoretically 

 maximum degree. A species may be regarded as the mean 

 of innumerable selective tendencies, each dragging it in 

 different directions. In the unstable and unfriendly world it 

 must make the best of a bad job, and must submit to many 

 compromises. A definition of adaptation in consonance with 

 this conception has recently been supplied by Fisher (1930, 

 p. 38), who says : ' Any simple example of adaptation, 

 such as the lengthened neck and legs of the giraffes as an 

 adaptation to browsing on high levels of foliage, or the con- 

 formity in average tint of an animal to its natural background, 

 loses, by the very simplicity of statement, a great part of the 

 meaning the word really conveys. For the more complex the 

 adaptation, the more numerous the different features of con- 

 formity, the more essentially adaptive the situation is recognised 

 to be. An organism is regarded as adapted to a particular 



1 Cuenot (1925, p. 19) has used the term adaptation statistique in an entirely 

 different and, as it seems to us, inappropriate sense. 



