No. 629] 



:>ABWIN AND HYBRIDIZATION 



537 



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 



species? (la, 2:17). 



Darwin comments frequently in the ''Origin of Spe- 

 cies," upon the fact that the hybrids produced from re- 

 ciprocal crosses often differ in fertility, and that while 

 two species may be difficult to cross, there is no strict 

 l)arallelism between the difficulty of effecting the cross 

 and the degree of sterility of the hybrids resulting there- 

 from. 



As Darwin observes, differences in the results in respect 

 to the relative ease of making reciprocal crosses had been 

 l>reviously noted by Koelreuter, who found, after two 

 hundred trials, continued for eight years, that while Mira- 

 bilis jalapa could easily be fertilized by M. longifiora, 

 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 fun- 

 damental resemblance between this fact and the ease with 

 which reciprocal grafts can be made, wherein Darwin in- 

 stances the fact that the currant can, although with dif- 

 ficulty, be grafted upon the gooseberry, while the recip- 

 rocal graft can not be made. Certainly the woll-ostab- 

 lished facts of somatic segregation followiMl ]>> ucniiinal 

 "mutation" — so-called, should sufficientl>- iiulicatf that 

 the behavior of the somatic and of the re)»roiluctive cells 

 should not be regarded as being so sharply separated as 

 is usually done in genetic studies. At all events, the prob- 

 lem as to the reason for the relative differences in the re- 

 spective facility of making reciprocal crosses, as well as 

 the further one of the differences as in the case of imilc 

 and hinny, between the respective products of rocipiocal 

 crosses, are questions that have been Init very littli> inves- 

 tigated since Darwin's time, and dcinniid tlioi'ono-li ex- 

 ploration. 



Since the advent of MoiuK'Haii AxuWv- in IDdO, it has 



