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 2 shows 

 an increase in variability together with the production of forms inter- 

 mediate between the parent races and often different from the FI. 

 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 FI 

 are supposed to be due to " contamination " in the FI 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.) 



" . . . .1 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.) 



" . . . . 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 



