14 LAWS GOVERNING THE STERILITY [Chap. IX. 



which very many species can most readily be crossed; 

 and another genus, as Silene, in which the most perse- 

 vering 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. Many analogous facts could be 

 given. 



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

 amount of difference, in any recognisable character, 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 differ- 

 ences in every part of the flower, even in the pollen, in 

 the fruit, and in the cotyledons, can be crossed. An- 

 nual and perennial plants, deciduous and evergreen trees, 

 plants inhabiting different stations and fitted for ex- 

 tremely different climates, can often be crossed with 

 ease. 



By a reciprocal cross between two species, I mean 

 the case, for instance, of a female-ass being first crossed 

 by a stallion, and then a mare by a male-ass; these two 

 species may then be said to have been reciprocally 

 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 completely independ- 

 ent of their systematic affinity, that is of any differ- 

 ence in their structure or constitution, excepting in 



