3 i8 THE VARIATION OF ANIMALS IN NATURE 



has been amplified to give rise to adaptations and long- 

 sustained evolutionary episodes. We must suppose that the 

 exponents of this theory would refer such cumulative modi- 

 fication to the continuous pressure of the environment or of 

 progressive individual effort. 



From the long discussion on the origin of variation it will 

 be seen how questionable is even the hereditary transmission of 

 induced modification. Still more speculative is the question 

 how far such a process could have produced (a) the progressive 

 modification of whole populations, and (b) adaptations and 

 complex organs. In short, though individual change and even 

 some degree of local diversification might arise from this 

 cause, we do not think that it is likely to have been a major 

 evolutionary agency. 



II. 'Evolution by Hybridism.' — Lotsy's theory is dis- 

 cussed in Chapter II (pp. 25-27). In addition to the criticism 

 advanced there that it offers no account of the origin of new 

 hereditary material, it seems to us to be open to the same 

 objection as we have put forward in the previous section — 

 viz. that it provides no explanation of progressive adaptation 

 and modification. That some part of the variation seen in 

 local populations may be due to the permutation and com- 

 binations of the stock of hereditary material canalised by 

 isolation, is not to be doubted. But the theory needs to be 

 supplemented by other principles in dealing with the major 

 problems of adaptation. 



III. ' Chance Survival.' — It has been suggested or implied 

 by various writers that variant individuals, which owe their 

 peculiar characters to spontaneous mutations, can survive and 

 multiply without the aid of selection. This idea is in agree- 

 ment with de Vries' original ' mutation theory ' in so far as it 

 seeks to dispense with selection (and indeed with the inherited 

 effects of modification by the environment) ; but it differs from 

 it in its conception of the size of the evolutionary steps and of 

 the process of species transformation. 



This idea has never been seriously formulated as a theory of 

 evolution. It has, as it were, lurked in the back of various 

 writers' minds and is implicit in (e.g.) the writings of Bateson. 

 This writer, though sceptical of the ' creative ' role of Natural 

 Selection, conceded that selection is operative in some measure : 

 ' by the arbitrament of Natural Selection all [variations] must 



