Chap. VIII. LAWS OF STEEILITY. 257 



between species, and of the hybrids produced from 

 them, is largely governed by their systematic af- 

 finity. This is clearly shown by hybrids never having 

 been raised between species ranked by systematists in 

 distinct families ; and on the other hand, by very closely 

 allied species generally uniting with facility. But the 

 correspondence between systematic affinity and the 

 facility of crossing is by no means strict. A mul- 

 titude of cases could be given of very closely allied 

 species which will not unite, or only with extreme 

 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 Dianthus, 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 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. 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. 



