536 



THE AMERICAN NATURALIST 



[Vol. LIII 



One of the primary questions concerning crossing that 

 interested Darwin was the matter of sterility and fertility 

 in hybrids. Investigators before Darwin's time had been 

 to a considerable extent obsessed by the species question, 

 which crossing was supposed to solve. If a cross suc- 

 ceeded, or produced fertile offspring, it argued that the 

 parent forms were '^varieties." If the cross failed, or if 

 its offspring were sterile, it demonstrated that they were 

 ''species." With the sole exception of Sageret (2), none 

 of the earlier hybridists seems to have formed anything 

 like the unit-character conception, and with the sole ex- 

 ception of Naudin and Darwin, no scientific theory was 

 conceived of which might explain the modus operandi of 

 amphimixis in the case of hybrids. 



By Darwin, the question of hybridization, while indeed 

 for the most part, taken up more or less conventionally, 

 received, nevertheless, broader treatment. To begin with, 

 Darwin held that the inability of species to cross ''is 

 often completely independent of their systematic affinity, 

 that is, of any difference in their structure or constitution, 

 excepting their reproductive systems" (la, 2:14). 



So that, even as early as the writing of the "Origin of 

 Species," Darwin is seen to maintain that the susceptibil- 

 ity of plants to crossing stood in no relation to the degree 

 of their resemblance to either parent, and that "the facil- 

 ity of making a first cross between any two species is not 

 always governed by their systematic affinity or degree of 

 resemblance to each other" (la, 2:16). 



This fact, he adds, is demonstrated by the case of re- 

 ciprocal crosses, alluding here to the relative facility of 

 making the cross, according as the one or the other species 

 - 11 ^cd as the male or the female. 



< >. r;i-i(.iially he says there is "the widest possible dif- 

 iM-o in tlu' facility of effecting a union. The hybrids, 

 inuicover, ]n-odii(*od from reciprocal crosses, often differ 

 in fertility" {ihid.). 



Darwin again later in "Animals and I*iants under 

 Domestication," refers to the matter as follows: 



