556 BOTANY OF THE LIVING PLANT 



mechanisms. The subject is a difficult one and has hitherto received 

 insufficient attention. 



The hypothesis of Natural Selection, which was one of the pillars 

 of the original Darwinian theory, is quite consistent with modern 

 genetical views, provided that it is recognised, first, that selection 

 operates only within genetically mixed populations and has no effect 

 on those which are genetically uniform (" pure lines ") ; and, secondly, 

 that the hereditary variations that provide the material upon which Natural 

 Selection can operate arise by mutation, i.e. that it is mutants and not 

 modifications [see above, p. 569) which undergo selection. 



There remains the vexed question of adaptation. It can hardly be 

 disputed that a high degree of adaptation to a particular environment 

 is one of the most striking properties of living things. On the other 

 hand, it is impossible to believe that every individual mutation is in 

 itself adaptive ; indeed there is abundant evidence to the contrary. 

 In addition it is probable that some heritable characters are " neutral," 

 in that they do not affect the chances of survival of the individual 

 one way or the other, though caution is indicated in this connec- 

 tion in view of the pleiotropic action of many genes. Adaptation 

 may be thought of as arising by an " integration," into genotypes 

 possessing a definite survival value, of a succession of mutational steps 

 which are individually non-adaptive. It should be realised that the 

 number of possible gene-combinations within a species greatly exceeds 

 the number of individuals of that species, so that there is ample scope 

 for the origin of adaptations by a process of " trial and error," especi- 

 ally as the same mutation may occur on many separate occasions. 

 The " opportunism " of adaptation in certain evolutionary trends 

 (e.g. in the evolution of the pollen-tube from a rhizoid to a gamete- 

 carrier) is suggestive. It must however be admitted that it is not 

 possible at present to offer a satisfactory explanation of the origin of 

 adaptation in general. 



In conclusion, it may be worth while to quote a summary of Sewell 

 Wright's views on Evolution as envisaged by a geneticist : " The 

 most general conclusion is that evolution depends upon a certain 

 balance among its factors. There must be gene mutation, but an 

 excessive rate gives an array of freaks, not evolution ; there must be 

 selection, but too severe a process destroys the field of variability, 

 and thus the basis for further advance ; prevalence of local inbreeding 

 within a species has extremely important evolutionary consequences, 

 but too close inbreeding leads merely to extinction. A certain amount 

 of cross-breeding is favourable, but not too much. In this dependence 



