JENNINGS: CHANGES IN HEREDITARY CHARACTERS 287 



Furthermore, we discover in our survey that there are at least 

 two well-marked controversies in flame at the present time: 



First, we have the general controversy between, on the one 

 hand, those who are mutationists and adherents of the strict 

 genotype view; on the other hand, those who, like Castle, be- 

 lieve that we observe continuous hereditary variations in the 

 progress of biparental reproduction. The mutationists attempt 

 to show that the apparent gradual modification of characters 

 observed in breeding is in reality a mere working out of Mende- 

 lian recombinations. Here we have contributions by Morgan 

 (1916), Pearl (1916, 1917), MacDowell (1916), Hagedoorn (1914), 

 and others on the one hand; while the full brunt of the attack 

 is borne on the other side by Castle. 



Second, we have a somewhat less lively controversy be- 

 tween the genotypic mutationists and the paleontological up- 

 holders of evolution by continuous variation. Echoes of this 

 we find in recent publications by Osborn and by Morgan. 



Now let us look briefly into the points at issue in the contro- 

 versy between the "genotypic mutationsts" and the upholders 

 of gradual change during biparental inheritance. 



Castle finds that in rats he can, by selection, gradually in- 

 crease or decrease the amount of color in the coat, passing by 

 continuous stages from one extreme to the other. As to this, 

 he holds two main points: 



1 . The change is an actual change in the hereditary character- 

 istics of the stock; not a mere result of the recombination of 

 Mendelian factors. This is the general and fundamental point 

 at issue. 



2. More specifically, he holds it to be an actual change n a 

 single unit factor; this single factor changes its grade in a con- 

 tinuous and quantitative manner. 



On the other side, the critics of these views maintain that the 

 changes shown are not actual alterations in the hereditary con- 

 stitution at all, but are mere results of the recombinations of 

 Mendelian factors. And specifically, they find a complete 

 explanation of such results as those of Castle in the hypothesis 

 of multiple modifying factors. 



