HYBRIDISM 231 



difficulty ; and on the other hand of very distinct 

 species which unite with the utmost facility. In the 

 same family there may be a genus, as Diantbus, in 

 which very many species can most readily be crossed ; 

 and another genus, as Silene, in which the most 

 persevering efforts have failed to produce between 

 extremely close species a single hybrid. Even within 

 the limits of the same genus, we meet with this same 

 difference ; for instance, the many species of Nicotiana 

 have been more largely crossed than the species of 

 almost any other genus ; but Gartner found that N. 

 acuminata, which is not a particularly distinct species, 

 obstinately failed to fertilise, or to be fertilised by, no 

 less than eight other species of Nicotiana. Very many 

 analogous facts could be given. 



No one has been able to point out what kind, or 

 what amount, of difference in any recognisable char- 

 acter is sufficient to prevent two species crossing. It 

 can be shown that plants most widely different in habit 

 and general appearance, and having strongly marked 

 differences in every part of the flower, even in the 

 pollen, in the fruit, and in the cotyledons, can be 

 crossed. Annual and perennial plants, deciduous and 

 evergreen trees, plants inhabiting different stations and 

 fitted for extremely different climates, can often be 

 crossed with ease. 



By a reciprocal cross between two species, I mean the 

 case, for instance, of a stallion-horse being first crossed 

 with a female-ass, and then a male-ass with a mare : 

 these two species may then be said to have been recip- 

 rocally crossed. There is often the widest possible 

 difference in the facility of making reciprocal crosses. 

 Such cases are highly important, for they prove that 

 the capacity in any two species to cross is often com- 

 pletely independent of their systematic affinity, or of 

 any recognisable difference in their whole organisation. 

 On the other hand, these cases clearly show that the 

 capacity for crossing is connected with constitutional 

 differences imperceptible by us, and confined to the 

 reproductive system. This difference in the result of 



