224 PLANT HYBRIDIZATION BEFORE MENDEL 



tinued over a period of eight years, that, while Mirabilis jalapa 

 could easily be fertilized by M. longiflora, the reverse cross could 

 not be effected. With regard to the difference in the facility with 

 which reciprocal crosses can be made, there may be some funda- 

 mental resemblance between this fact and the ease with which 

 reciprocal grafts can be made, wherein Darwin instances the fact 

 that the currant can, although with difificulty, be grafted upon 

 the gooseberry, while the reciprocal graft cannot be made. Cer- 

 tainly the well-established truth of factorial mutations in vegeta- 

 tive cells, followed by germinal differences to correspond, should 

 sufficiently indicate that the behavior of the somatic and of the 

 reproductive cells ought not to be regarded as being so sharply 

 separated as is usually done. At all events, the problem of the 

 reason for the relative difference in the respective facility of mak- 

 ing reciprocal crosses, as well as the further one of such differ- 

 ences as exist, in the case of mule and hinny, between the re- 

 spective products of reciprocal crosses, are questions that have 

 been too little investigated since Darwin's time, and require ex- 

 planation. 



Since the advent of Mendelian studies in 1900, it has been 

 rather conventionally assumed that reciprocal crosses are more 

 or less identical in type. That such is not necessarily the case, 

 Darwin's early observations should suffice to indicate. 



The problem of the fertility of selfed and crossed plants en- 

 gaged Darwin's close interest. In forty-one 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 100:60. 

 In another experiment to determine the relative fertility of flow- 

 ers when crossed or selfed, the ratio in thirty cases, belonging to 

 twenty-seven species, was as 100:55". 



There is no evidence, Darwin finds, 



". . . that the fertility of plants goes on diminishing in successive self- 

 fertilized generations," and "no close correspondence, either in the parent 

 plants or in the successive generations, between the relative number of 

 seeds produced by the crossed and self-fertilized flowers, and the relative 

 powers of growth of the seedlings raised from such seeds." (lb, p. 327.) 



Darwin's investigations were directed quite extensively to the 

 question of self-sterility in plants, a field which bears strongly 



