OTHER THEORIES OF EVOLUTION 343 



analogy is admitted, we have still to account for how this 

 emancipation becomes characteristic of populations. 



Summary of the Various Theories oe Orthogenesis 



As there are so many different kinds of phenomena which 

 have been looselv included under this head, and as the various 

 theories seek to explain different manifestations of evolution, 

 we cannot easily deal with the subject comprehensively. 

 In general, however, three theories cover the principal array 

 of phenomena : 



(1) It is held that some selective effect (either direct or 



indirect) explains a considerable part of the facts. 



(2) (a) The phenomena of excessive growth are explained 



by some by reference to abnormal physiological 

 processes analogous to individual defects. 

 (b) Internal physiological processes (racial life-cycle) 

 are held responsible for the process of recapitulation. 



(3) Environmental pressure is deemed to be effective 



either by acting upon a limited range of variability 

 or by maintaining or releasing normal physiological 

 inhibitions. 



We believe that none of these theories is in any way 

 near to being proved. In fact, as far as rigorous proof is 

 involved none can rank as more than a plausible suggestion. 

 (1) and (2) (a) have more in support of them than the others, 

 though the selective theories depend entirely on the assump- 

 tion that selection is a vera causa, and to utilise (2) (a) as a theory 

 by which the multiplication of variants is effected involves us 

 in some very grave difficulties. It would be possible to expand 

 the concept of a physiological momentum to include other, 

 perhaps all, evolutionary phenomena. Indeed a very great 

 variety of structures and habits impress themselves on us in 

 this way, viz. as the product of non-adaptive tendencies 

 arising within the organism itself. Nevertheless we have as 

 yet no positive evidence as to how such changes come to 

 characterise whole populations. On the other hand, the 

 selective theories supply us, theoretically at least, with an 

 explanation of both the change and its spread. 



V. Theories of Bergson and others. — There remain 

 for consideration certain speculations and theories that cannot 



