MENDEL'S FIRST LAW 37 



sive advances may belong to a different category. This 

 may be a serious question for the evolutionist, but has 

 nothing to do with the problem that concerns us here. 



In recent years an entirely unexpected and important 

 discovery in regard to segregating pairs of genes (allelo- 

 morphs) has been made. In an ever-increasing number of 

 cases it has been found that there may be more than 

 two distinct characters that act as allelomorphs to each 

 other. For example, in mice, yellow, sable, black, wliite- 

 bellied gray, and gray-bellied gray (wild type) are allelo- 

 morphs, i.e., any two may be present (as a pair) in an 

 individual, but never more than two. In DrosopJiila the 

 eye colors white, eosin, cherry, blood, tinged, buff, milk, 

 ivory, coral and the normal allelomorph form a series of 

 multiple allelomorphs. In the grouse locust, Paratettix, 

 there are nine types that may be allelomorphic, all of 

 which exist in the wild state (Nabours). In Drosophila, 

 again, there are as many as twelve other series of allelo- 

 morphs known at present ; in rats there is a small allelo- 

 morphic series, also two in guinea pigs and two in rabbits. 

 In plants there are a few cases known, especially in corn. 

 In all these series it is the same organ that is mainly 

 affected by the different allelomorphs, which seems ''natu- 

 ral, ' ' but was not necessarily to have been expected. The 

 chief interest of these series is that they appear to demon- 

 strate that the normal (wild type) allelomorph, and its 

 mutant mates need not be due to presence and absence, 

 but rather represent modifications of the same unit in the 

 hereditary material ; for, taken literally, only one absence 

 is thinkable, and yet in Drosophila there are eight such 

 "absences'' in one series. 



As has been stated, Mendel did not make it clear that 

 there exists in the normal animal or plant the same dual- 

 ity that comes to light when a hybrid is produced ; never- 

 theless this condition is implied, at least, in his paper, 

 and has been taken for granted in practically all of the 

 modern w^ork on heredity. The demonstration that such 



