270 THE TREND OF THE RACE 



we have made practically no progress in ascertaining, in any form, 

 whatever relation may exist between the nature of the variation 

 and the kind of external stimulus by which it is evoked. 



A large part of the congenital variations that appear in organ- 

 isms are mere products of the mingling of factor differences con- 

 tained in the germ cells of the parents. Where such variations 

 are not obviously the expression of typical Mendelian inheritance 

 they are frequently expHcable as unusual factor combinations 

 which are nevertheless essentially Mendelian. Certain variations 

 may perhaps be attributable to the loss of factors and others to 

 the reduplication of one or more factors, as the result of some 

 anomalous behavior of the germ plasm, such as occurred in the 

 mutant (Enothera gigas and several other similar cases. But all 

 such variations as these are probably of minor significance in 

 relation to the general problem of progressive evolution. They 

 are the results of the shufSng of the cards, and at best they can 

 produce only new combinations of old factors. 



There are writers (Lotsy, Hagedoom) who hold that the kinds 

 of variations just alluded to are the only ones of which we have 

 any evidence. But if we admit the existence of this kind of 

 variability only, we are landed in serious difficulties. There is 

 certainly no adequate reason for denying that variation is a real 

 phenomenon dependent upon qualitative changes in the germ 

 plasm. Many cases are known in which the appearance of new 

 mutants is in all probability dependent upon such qualitative 

 germinal variations. But with few exceptions their occurrence 

 seems entirely fortuitous and we can form no conjecture as to 

 their possible cause. 



There is a certain amount of experunental evidence that 

 germinal modifications may be evoked by environmental agen- 

 cies. The experiments of Tower on the production of mutants in 

 the Colorado potato beetle, and the work of MacDougal and 

 Gager in inducing mutations in (Enothera and other plants by 

 salt solutions and radium are among the few investigations on 

 multicellular organisms which have yielded positive indications of 

 germinal response to changes in the environment. 



