PLANT HYBRIDIZATION BEFORE MENDEL 223 



biological units, and, with the sole exception of Naudin and 

 Darwin, no scientihc theory was even 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 neverthe- 

 less broader treatment. To begin with, Darwin held that the in- 

 ability of species to cross 



". . . is often completely independent of their systematic affinity, that is 

 of any difference in their structure or constitution, excepting in 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 susceptibility of plants to 

 crossing stood in no necessary relation to the degree of their re- 

 semblance, and that 



". . . facility 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 reciprocal 

 crosses, alluding here to the relative facility of making the cross, 

 according as the one or the other species is used as the male or 

 the female. 



"Occasionally," he says, there is "the widest possible difference, in the 

 facility of effecting a union. The hybrids, moreover, produced from 

 reciprocal crosses, often differ in fertility." {ib.) 



Darwin again later, in "Animals and Plants under Domestica- 

 tion," refers to the matter as follows : 



"why should some species cross with facility, and yet produce very 

 sterile hybrids ; and other species cross with extreme difficulty, and yet 

 produce fairly fertile hybrids'? Why should there often be so great a 

 difference in the result of a reciprocal cross between the same two 

 species*?" {ib., p. 217.) 



Darwin comments frequently, in the "Origin of Species," upon 

 the fact that the hybrids produced from reciprocal crosses often 

 differ in fertility, and that, while two species may be difficult to 

 cross, there is no strict parallelism between the difficulty of effect- 

 ing the cross, and the degree of sterility of the hybrids resulting 

 therefrom. 



As Darwin observes, difference in the results, in respect to the 

 relative ease of making reciprocal crosses, had been previously 

 noted by Kolreuter, who found, after two hundred trials con- 



