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 F, has an increased size due to vigor produced by 
crossing and not due to heredity at all. This increased size persists partially 
in F,, but for the most part is not in evidence beyond F;. 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 F, and the subsequent reversed regression in the second F».”’ 
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 F,, 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, F, 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 F;. 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 
