140 THE SHORTER SCIENTIFIC PAPERS 



McDowell, Mulier and others equally insistent that genes 

 once having originated pass on from one generation to 

 another unchanged except in comparatively rare instances 

 where so-called "mutations" occur.^ It is maintained by 

 those advocating this view that the results in connection 

 with hooded rats on which Castle bases his contentions, 

 are due to an uncertain number of modifying genes not in 

 themselves variable, and that the existence of such genes 

 has been demonstrated in other organisms presenting re- 

 sults similar to those obtained in rats. The work of Little 

 (1917) with mice where three segregating types of spot- 

 ting were found to produce varying degrees of color 

 pattern, indicates that multiple genes are involved. Fur- 

 thermore, the analysis by Little of the data obtained by 

 Castle, Phillips and Wright, points decidedly to the in- 

 terpretation of their results on the basis of multiple genes 

 instead of a continually varying gene. 



It would thus appear evident that the theory outlined 

 by Castle is open to quite the same objections that occur 

 in connection with the mutation theory of DeVries, and 

 that there is little evidence for believing that it has any 

 fundamental value in explaining evolution. 



The mutation theory of Morgan and his associates, 

 based primarily on results obtained in studies of the small 

 "fruit-fly" Drosophila, apparently presents quite another 

 view of the subject. Here it is clearly indicated that 

 evolution has taken place through the incorporation of 

 mutant changes, and that these changes are due to dis- 

 continuous "mutations" of genes as exemplified in mul- 

 tiple allelomorphs. 



Assuming the validity of the arguments based on link- 

 age relations in respect to the localization of the genes, 



Jennings (1917) has recently endeavored to show that the views of Castle and 

 his opponents are identical. This, however, is by no means the case. On the one 

 hand there is the idea of a continually variable gene (coat-color-producing gene in 

 rats) moved gradually along a given scale by selection. On the other hand there is 

 the idea of a rarely mutating gene {e. g., sex-limited eye color producing gene in 

 Drosophila) moving abruptly from one part to another of the scale. Its position 

 once obtained remains for a long time constant. These differences of interpretation 

 are at present irreconcilable. 



Since this note was written, Morgan (1917) has discussed the matter in detail, 

 presenting arguments quite similar to those mentioned above, and arriving at a similar 

 conclusion. 



