Mutation 117 



(lone so in the past. 'J'he tetraploid form has more doses of desir- 

 able growth factors than had its diploid ancestor, and as a rule is 

 visibly more vigorous in one respect or another. A theoretical 

 limitation, however, applies here also. Tetrai)loidy involves 

 merely a quantitative gain, a multiplication of genes already 

 present. One feels that not much progressive evolution could 

 take place without the appearance of qualitatively new genes and 

 the production of distinctly new adaptive characters. 



It would be safe to state that the foregoing considerations have 

 caused many biologists to feel less certain in explaining evolution 

 than they were a few years ago. This loss of faith in mutation, 

 taken together with recent discoveries on inheritance of acquired 

 characters (see chap, ii), is causing many to seek an explanation 

 of progressive evolution in Lamarckian terms. 



It is of some interest to note that the original ''classic" 

 examples of mutation provided by De Vries are probably not gen- 

 uine cases. It had long been suspected by some that Oenothera 

 Lamarckiana was a hybrid and its ''mutants" merely extracted 

 recessives, but it was difficult to account on this basis for the very 

 small number of "mutants" that were thrown every generation. 

 MuLLER (7) probably deserves the credit for solving this vexing 

 problem. In the fruit fly he discovered an essentially true- 

 breeding hybrid race and explained it by a system of balanced 

 lethal factors. These factors assert their lethal effect only when 

 they occur in the homozygous recessive condition. In this race 

 of flies, two such factors are present in heterozygous condition on 

 the same pair of chromosomes, the dominant members of the 

 heterozygous sets being on the opposite chromosomes of the pair. 

 Such a hybrid continues to breed true as such, since any attempt 

 to segregate brings the homozygous recessive condition of one or 

 the other lethal with resulting death to the progeny. The reces- 

 sives of any heterozygous set of genes on this same chromo- 

 some pair will remain concealed when this stock is allowed to 

 inbreed. Occasional crossing over will cause the appearance of a 

 few of these recessives (in predictable frequencies), like the "mu- 

 tants" thrown by Oe. Lamarckiana. 



In fact, De Vries himself now subscribes (5) to an explanation 

 fundamentally similar to the preceding. About one-half of the 



