MENDELISM AND OTHER METHODS OF DESCENT 23 1 



As Mendelism may be considered a somewhat weaker form 

 of sex-inheritance, so there are weak forms of Mendelism which 

 serve to connect the definitely polarized forms of alternative 

 descent with the other still less specialized methods of inheri- 

 tance. Instead of the typical dominance of one parental char- 

 acter in tiie conjugate phase, there may be an intermediate or 

 compromise, but this need not interfere with alternative descent 

 in the gametes of the next generation. Or it may happen that 

 the conjugate organism expresses a character shown by neither 

 of the gametes when mated with their own kind. Definite 

 alternative descent in the gamete progeny also grades off into 

 intermediate expressions of the two characters, or into the ex- 

 pression of a whole series of integraded intermediates, or some 

 ancestral characteristic may reappear, though long dormant in 

 both the parent varieties. 



The extent to which Mendelism and other less accentuated 

 forms of polarized inheritance exist in natural species is a sub- 

 ject which has received very little investigation thus far. The 

 reasonable expectation would be that species will be found to 

 differ very greatly in these respects, just as they show very great 

 diversity of degrees of accentuation of alternative sexual char- 

 acters. Narrow breeding, which tends to the accentuation of 

 variations, probably tends also to the accentuation of polarity of 

 expression. This agrees with the fact that Mendelism is so 

 frequent a phenomenon in crosses of narrow-bred varieties, but 

 so rare in crosses of undomesticated species or subspecies. A 

 course of narrow breeding, such as experimenters in Mendelism 

 prescribe as a means of ascertaining the " purity" of the stock 

 by securing uniformity might well bring the plants or animals 

 into condition for an exhibition of strict Mendelism, instead of 

 showing somewhat less specialized forms of polar inheritance. 

 Mutative variations which represent reactions from abnormal 

 uniformity may be ready from the first to make the most of the 

 new diversity by means of polar inheritance. Nor is it without 

 significance for general evolutionary purposes that the natural 

 selections of a dominant variation would not tend toward uni- 

 formity, but preserve and increase the heterism of the species. 

 It is only by highly conscious selection of the "pure domi- 



