HYBRIDISM 247 



crossed under the same circumstances; it depends in part upon 

 the constitution of the individuals which happen to have been 

 chosen for the experiment. So it is with hybrids, for their degree 

 of fertility is often found to differ greatly in the several individuals 

 raised from seed out of the same capsule and exposed to the same 

 conditions. 



By the term systematic affinity is meant, the general resem- 

 blance between species in structure and constitution. Now the 

 fertility of first crosses, and of the hybrids produced from them, 

 is largely governed by their systematic affinity. 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 multitude 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 in- 

 stance, 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 fertilize, or to be fertilized 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 recognizable 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 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 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 differ- 

 ence in the facility of making reciprocal crosses. Such cases are 



