SPONTANEOUS VARIATIONS 43 



more than one interpretation. The problem, in fact, is too 

 complex and difficult to be solved by observing differences 

 between parents and children, and then trying to trace 

 them to forces from the environment acting on the former. 

 We cannot solve it even by observing a short line of indi- 

 viduals. We must turn as usual to the evidence afforded by 

 evolution. 



72. All the processes of adaptive racial change may be 

 classed under one of two categories. Either they are progres- 

 sive, or else they are regressive. Progressive evolution 

 depends on progressive variations, and leads to increased 

 magnitude or complexity of structure (and function) or both. 

 Regressive evolution depends on regressive variations, and 

 leads to decreased magnitude or complexity. 1 Now it must 

 be noted that no influence from the environment of any sort, 

 however effective as a cause of variations, can, by itself, be a 

 cause of evolution, except as a rare coincidence. It might 

 by acting on a succession of generations lead to great racial 

 changes ; it could not lead to adaptive racial change. Indeed 

 if we reject the Lamarckian doctrine and suppose that evolu- 

 tion is due to Natural Selection, such an influence would 

 prevent adaptation. 



73. Suppose the influence were one which affected the 

 whole species for a prolonged period and which acted benefi- 

 cially on the individual (e. g. good and plentiful food), then, 

 presumably, the germ-plasm, if it underwent any change at all, 

 would share in the good fortune of the rest of the parent's 

 body, and the variations of the child would be of a progressive 

 nature for example, increased size and vigour. But in- 

 creased size or vigour or any other progressive change, even if 

 it were a general change and not one confined to particular 

 structures or organs, would but rarely be of advantage to the 

 species. It would throw the race out harmony with its 

 environment, as by rendering necessary a larger supply of 

 food. In considering these problems the close, the extra- 

 ordinarily close, adaptation of every species in all its organs 

 and powers to its whole environment must constantly be borne 

 in mind. Consider, for instance, how wonderfully the various 

 species of insects are adapted to their several environments, 

 and how utterly unsuited any species is to the environ- 

 ment of any other. They fill particular niches in nature to 

 which they are closely fitted by all their characters. Species 

 undergo evolution because the environment undergoes 

 change ; because step by step, by slow adaptive alterations of 



1 See 8 97. 



