GENERAL DISCUSSION 67 



differences than Davenport seems to, although he finds some. These 

 are my differences due to "strain idiosyncrasies." 



The significance of my data as regards the pressing question of 

 the chief influences in species change seems to me to be that of pointing 

 toward the sudden appearance of definite discontinuous fixed differ- 

 ences either of the nature of new unit characters or of new combinations 

 of old unit characters, endowed from the start with taxonomic stabil- 

 ity, behaving in heredity as consistent alternative characteristics along 

 Mendelian lines. In other words it seems to me that my data indicate 

 the reality of mutations as real species differentiating characters. 

 The visible differences between hereditary strains of organisms based 

 on the accumulation of fluctuating variations by some method of selec- 

 tion may be even larger in appearance than the mutational differences 

 and yet lack the stability and hence fundamental reality of these latter 

 differences. Apparently, however, by some means they may come to 

 acquire the inheritance behavior and stability of the mutational differ- 

 ences. At least the cocoon differences in silk worms which are the 

 result of selection methods seem to be tending strongly toward the 

 acquirement of the same type of inheritance, viz., alternative Men- 

 delian inheritance, as that of the larval characteristics. If this con- 

 dition can be really attained then the differences will be as real and 

 species-distinguishing as those which arise as mutations. 



But I concede readily that my conclusions are not so inevitable 

 from my data as my expression of them would seem to indicate. And 

 I wish to leave with my readers no wrong impression of an overesti- 

 mate on my part either of the value of the data themselves or of the 

 worth of the few generalizing conclusions expressed in this paper. I 

 offer the data as facts as nearly as I can see and describe them, con- 

 tributing toward our gradually growing knowledge of inheritance 

 phenomena. 



