AN ANALYSIS OF THE EFFECT OF SELECTION. 39 
CONTAMINATION OF ALLELOMORPHS. 
When two races that differ in quantitative characters are crossed, 
it is frequently observed that F, is fairly uniform, and that F, shows 
an increase in variability together with the production of forms inter- 
mediate between the parent races and often different from the F,. 
There are two current methods of accounting for these cases: 
(1) The two races are assumed to have differed in a number of 
Mendelian factors affecting the character in question. The observed 
result is then explained as due to the recombinations of these factors. 
(2) The two races are assumed to have differed in only one factor 
affecting the character in question, and the new types observed in F, 
are supposed to be due to ‘‘contamination” in the F,; hybrid, that is, 
allelomorphs present in the heterozygote are supposed to have influ- 
enced each other, so that they do not come out unchanged. 
The fundamental principle of the first explanation—that more 
than one factor may influence the same character—is admitted by 
all Mendelians. But many of the adherents of that explanation are 
unwilling to admit that “contamination of allelomorphs” has ever 
been experimentally demonstrated. Let us then examine the evi- 
dence that is brought forward in support of that assumption. 
The following quotations are the chief ones bearing on the ques- 
tion that I have been able to find in recent literature: 
“The currently accepted explanation (of size inheritance), which its 
supporters choose to call ‘Mendelian,’ rests upon the idea of gametic purity 
in Mendelian crosses. It assumes that Mendelian unit-characters are un- 
changeable and unvarying, and that when they seem to vary this is due to a 
modifying action of other unit-characters (or factors) . . .. The idea 
of unit-character constancy is a pure assumption. In numerous cases unit- 
character inconstancy has been clearly shown, as in the plumage and toe 
characters of poultry according to the observations of Bateson and Daven- 
port, and the coat-characters and toe-characters of guinea-pigs in my own 
observations. Unit-character inconstancy is the rule rather than the ex- 
ception.” (Castle, 19166, p. 209.) 
I have shown in numerous specific cases that when unlike 
gametes are brought together in a zygote they mutually influence each other; 
they partially blend, so that after separation they are less different than they 
were before. The fact remains to be accounted for that partial blending does 
occur (1) when polydactyl guinea-pigs are crossed with normals (Castle, 
1906); (2) when long-haired guinea-pigs are crossed with short-haired ones 
(Castle and Forbes, 1906); and (3) when spotted guinea-pigs or rats are 
crossed with those not spotted (MacCurdy and Castle, 1907). Davenport 
has furnished numerous instances of the same thing in poultry; indeed, he has 
shown that ‘‘imperfection of dominance”’ and of segregation are the rule rather 
than the exception in Mendelian crosses in poultry.”’ (Castle, 1916d, p. 253.) 
cs . The English unit-character had changed quantitatively in trans- 
mission from father to son. This seems to us conclusive evidence against 
the idea of unit-character constancy, or ‘gametic purity.’”’ (Castle and 
Hadley, 1915.) 
: We are often puzzled by the failure of a parental type to reappear 
in its completeness after a cross—the merino sheep or the fantail pigeon, for 
