50 AN ANALYSIS OF THE EFFECT OF SELECTION. 



view. Castle has met this objection in the following manner (Castle 

 and Wright, 1916) : 



"This suggests the idea that that loss (of 'plus' character) may have been 

 due to physiological causes non-genetic in character, such as produce in- 

 creased size in racial crosses; for among guinea-pigs (as among certain plants) 

 it has been found that Fi has an increased size due to vigor produced by 

 crossing and not due to heredity at all. This increased size persists partially 

 in Fo, but for the most part is not in evidence beyond Fi. I would not sug- 

 gest that the present case is parallel with this, but it seems quite possible 

 that similar non-genetic agencies are concerned in the striking regression of 

 the first F2 and the subsequent reversed regression in the second F2." 



This comparison seems to me to be rather far-fetched, and I am 

 quite unable to understand the hypothesis of ''non-genetic physiologi- 

 cal causes." That they are "physiological" is, of course, obvious; 

 but they depend for their appearance on the pedigree of the animal, 

 and they are persistent to F2, so why "non-genetic"? The results 

 from size crosses are entirely explicable on the basis of Mendelian 

 modifying factors, so why need one appeal to vague "non-genetic," 

 yet transmissible, factors? And is not such an appeal, in principle, 

 an appeal to modifying factors? It certainly involves the assump- 

 tion that the grade depends on transmissible material other than the 

 hooded factor itself. 



In the tenth generation of Castle's plus selection series there ap- 

 peared two rats of considerably higher grade than any individuals 

 of that series previously recorded. These individuals were shown 

 (Castle and Phillips, 1914, pp. 26-31) to differ from the plus race by 

 a single dominant factor. This has been taken by MacDowell to 

 indicate that a new modifying factor arose by mutation. But Castle 

 has now presented evidence indicating that the mutation occurred 

 in the hooded locus itself. When homozygous "mutants " were crossed 

 to wild rats, F2 consisted in self-colored rats and rats of the same grade 

 as the mutant series — no hooded individuals. (Castle and Wright, 

 1916.) Castle (1916) concludes from this evidence: "This serves 

 to confirm the general conclusion that throughout the entire series 

 of experiments with the hooded pattern of rats we are dealing with 

 quantitative variations in one and the same genetic factor." Now, 

 the "mutant" variation differs from the other results obtained by 

 Castle in two respects: It appeared suddenly, as a definite and very 

 slightly variable character, and it fails, when crossed to self, to give 

 normal hooded in F2. Because of the first point, it is probable that 

 it arose during the experiment as a new variation; because of the sec- 

 ond, it is probable that it is a variation in the hooded factor itself. 

 Since these conclusions as to its nature are based entirely on the points 

 in which it differs from the remainder of the results, it is difficult to 

 see how Castle's case for these results is in any way improved. On 

 the contrary, if this is the behavior to be expected of a new variation 



