CONCLUSIONS 373 



Some authors have nevertheless insisted that these phenomena 

 are due to an internal impulse, and indeed the various theories 

 [cf. Chapter VIII) by which it is sought to explain them as 

 due to Natural Selection alone, or to selection combined with 

 heterogony, are subject to the same general criticism as the 

 selection theory. Analogy with physiological and pathological 

 processes justifies us to some extent in a belief in an internal 

 directive force, though the proof of its existence depends 

 rather on the exclusion of other causes than on the direct 

 demonstration of such a principle. 



If it was correct to exclude other causes and to inter- 

 pret the facts of orthogenesis as indicative of an internal 

 potential, it would be possible to suggest a theoretical account 

 of the origin of adaptations. We might assume that such a 

 momentum affecting functionally associated parts could exert 

 an organising influence on a part or on the whole animal, 

 and even that, by what we might describe as a functional 

 quickening, it could promote and attract to itself the kinds of 

 mutations required in any adaptive situation. But for such a 

 suggestion, of course, we have little evidence, and its chief 

 justification is the poverty of the other theories. 



If the estimation of the various theories just presented is 

 a fair one, we are plainly left with a negative result and the 

 inference that our knowledge is too defective to provide an 

 answer. We may, perhaps, claim to have shown that group 

 formation is, in part at least, independent of Natural 

 Selection ; that the effect of the environment alone cannot give 

 rise to adaptations; and that Natural Selection cannot be 

 excluded from the possible causes of adaptations, though it is 

 more likely to have produced specialisation than the more 

 fundamental processes of organisation. 



Against this scepticism and uncertainty we are entitled to 

 set certain impressions. It seems that organisation in its 

 more fundamental manifestations, especially in development, 

 is something for which the activities of Natural Selection, even 

 if estimated in the most generous fashion, cannot well account. 

 With more evidence, and particularly more knowledge of 

 bionomics, it might be shown that selection does, in fact, 

 produce certain kinds of specialisation. We find it hard to 

 believe either that the ascertained ' fit ' of the organism to its 

 environment could enable selection to work with the necessary 



