CHAPTER VIII 

 THE MUTATION THEORY 



The nature of mutations The impossibility of distinguishing between 

 fluctuations and mutations by means of experiment The alleged immutability 

 of mutations Our power of studying smaller variations Human inter-varietal 

 crossings Latent characters are common when domestic varieties cross ; they are 

 rare when natural varieties cross The effects of Natural and Artificial selection 

 are not identical Objections to the mutation theory The distinction between 

 inheritance and reproduction The modes in which mutations are inherited and 

 reproduced. 



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285. "^^ T'HETHER due to unit segregation and gametic 

 purity, or to patency and latency of the components 

 of compound allelomorphs, there can be no doubt 

 of the actual occurrence of the Mendelian phenomena. We must 

 endeavour, therefore, to estimate the part played by them in nature. 

 The more extreme section of the experimental school insists that 

 all those differences (even minute differences) between parents and 

 children which we have defined as variations (innate or germinal 

 differences, differences in hereditary tendencies) are mutations, 

 that the inheritance of all mutations is Mendelian, and that muta- 

 tions are permanent changes which" selection alone can eliminate," 

 whereas ' continuous,' * normal,' or ' fluctuating ' variations are mere 

 modifications due to changes in the action of the environment. Con- 

 sequently, according to them, all evolution, at any rate all permanent 

 and considerable evolution, is founded solely on mutations. It 

 follows if this hypothesis be true, that the stability of racial 

 characters is due, not to their long-continued selection, but to the 

 circumstance that they have arisen through the accumulation of 

 mutations, all of which were stable from the first. 1 



1 " Doubtless some of the so-called fluctuations are in reality small mutations, 

 whilst others are due to environmental influence. The difficulty of distinguishing 

 between the two is very great. The simultaneous existence of small mutations 

 and large fluctuations leads to the disguising of the former by the latter. Only 

 careful and laborious analysis will avail us here, and such analysis is precisely 

 what is at present lacking. The position is roughly as follows. Of the inheritance 

 of mutations there is no doubt. Of the transmission of fluctuations there is no 

 very strong evidence. It is therefore reasonable to regard the mutation as the 

 main, if not the only basis of evolution. And the great service which Mendel has 



