ii6 Outline of Genetics 



dubious as a basis for explaining evolution; but mutation, with 

 natural selection among the mutants, will doubtless account for 

 most of the facts. Now, in view of the more accurate knowledge 

 of the mutation phenomenon that has been developed in recent 

 years, the adequacy of mutation in explaining evolution must be 

 considered more critically. 



First of all, it is evident that "complexmutation," "defi- 

 ciency," and "duplication" could have played no important part 

 in evolution, merely on account of the extreme rarity of these 

 phenomena if for no other reason. Locus changes are sufficiently 

 common, but consider the quahty of the mutants which result! 

 In practically all cases the change is a "loss" mutation, and surely 

 evolution cannot be accounted for on such a basis! One might 

 merely regard this as evidence of the "trial and error" method by 

 which nature operates, only rarely making those "gains" which 

 must serve as the basis of progressive evolution. A few "gain" 

 mutations have been reported, but there is reason to suspect that 

 even these may be merely "reverse" mutations, regaining that 

 which had previously been lost. Furthermore, the locus changes 

 that have been reported, be they losses or gains, have seemed 

 consistently non-adaptive. In short, it is difficult to imagine how 

 progressive evolution can be accounted for either through single 

 locus changes or through the accumulation of numerous locus 

 changes. One can readily admit that such changes may account 

 for the multiphcation of varieties or even species "on the same 

 level," but can hardly be convinced that "our larger phylogenetic 

 edifices have been erected from such building blocks." It is quite 

 likely, however, that our knowledge is still too limited to visualize 

 the evolution of the ages in terms of what we have seen happening 

 during a very few years. 



Non-disjunction is out of the question as a basis for evolution- 

 ary progress. The resulting "unbalanced" forms are clearly 

 abnormal, and it is very doubtful whether they could permanently 

 perpetuate themselves under the most favorable conditions, much 

 less survive under conditions of sharp competition and environ- 

 mental stress. 



Tetraploidy might well account for a certain amount of evolu- 

 tionary progress, and we have good evidence that it has actually 



