14 LAWS GOVERNTSTG THE STEEILITY [Chap. IX. 



in 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 Xicotiana have been 

 more largely crossed than the species of almost any 

 other genus ; but Gartner found that X. acuminata, 

 which is not a particularly distinct species, obstinately 

 failed to fertilise, or to be fertilised by no less than 

 eight other species of Xicotiana. Many analogous facts 

 could be given 



Xo 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 stronglv marked differ- 

 ences 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 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 *hen 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 

 independent of their systematic affinity, that is of any 

 difference in their structure or constitution, excepting 



