PLANT HYBRIDIZATION BEFORE MENDEL 237 



parents belonged to different "species," whereas fertility indicated 

 that the parents were varieties of the same species. At present, 

 while it is recognized that organisms more distantly related — ■ 

 frequently different so-called "species" — do not ordinarily cross, 

 or, if they do, the progeny are quite frequently sexually sterile, 

 yet the attitude has quite changed from the strictly systematic 

 point of view formerly adhered to. Darwin recognized in general 

 that : 



"There is often the widest possible difference in the facility of mak- 

 ing reciprocal crosses. Such cases are highly important, for they prove 

 that the capacity in any two species to cross is often completely inde- 

 pendent of their systematic affinity, that is of any difference in their 

 structure or constitution, excepting in their reproductive systems." (la, 

 2: 14.) 



Darwin's relation to the study of hybridization is, as already 

 stated, chiefly known through his extensive and classical experi- 

 ments on self and cross-fertilization in plants. 



In forty cases, belonging to twenty-three species, the ratio of 

 the fertility of the crossed to that of the self-fertilized plants was 

 found to be as 100:50 {ib., pp. 314-17); in another case, the 

 ratio, in thirty cases, belonging to twenty-eight species, was as 



100:75. (2'^-' PP- 322-3.) 



Darwin, at the outset, merely comments on the results of cross- 

 ing as follows : 



"In considering the final result of the commingling of two or more 

 breeds, we must not forget that the act of crossing in itself tends to 

 bring back long-lost characters not proper to the immediate parent- 

 forms." (ic, 2*: 64.) 



It was noticed that from three to eight generations were usually 

 required before a breed derived from a cross comes to be con- 

 sidered free from danger of "reversion." What constituted the 

 machinery to bring about reversion remained, but for Mendel's 

 as yet undiscovered researches, unknown. The state of knowledge 

 in this regard is exemplified by Darwin's remark : 



"That the act of crossing in itself gives an impulse towards reversion, 

 as shown by the re-appearance of long-lost characters, has never, I be- 

 lieve, been hitherto proved." (ic, 2:13.) 



Darwin recognized, as did most of the breeders before Mendel, 



that: 



"As a general rule, crossed offspring in the first generation are nearly 



