242 LAWS GOVERNING THE STERILITY Chap. VIIL 



most persevering efforts have failed to produce between ex- 

 tremely-close species a single hybrid. Even within the limits 

 of the same genus, wc 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 Giirt- 

 ner found that N. acuminata, which is not a particularly dis- 

 tinct species, obstinately failed to fertilize, or to be fertilized 

 by, no less than eight other species of Nicotiana. Many analo- 

 gous facts could be given. 



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

 amount, of difference in any recognizalile character is suflicient 

 to prevent two species crossing. It can be shown that plants 

 most Avidely 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 ever- 

 green trees, plants inhabiting difl'erent 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 fe- 

 male-ass, and then a male-ass with a mare : 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 tliat the capacity in any two species to cross is often 

 completely independent of their systematic affinity, or of any 

 difference in their whole organization, except in their rej^ro- 

 ductive systems. The diversity of result in reciprocal crosses 

 between the same two species was long ago observed by Kiil- 

 reuter. To give an instance : Mirabilis jalapa can easily be 

 fertilized by the pollen of M. longiflora, and the hybrids thus 

 25roduced are suiliciontly fertile ; but Kolreuter tried more than 

 two hundred times, during eight following j-ears, to fertilize 

 reciprocally M, longiflora v»itli the pollen of ^I. jalapa, and 

 utterly failed. Several other equally striking cases could be 

 given. Thuret has observed the same fact with certain sea- 

 weeds or Fuci. Gartner, moreover, found that this difference 

 of facility in making reciprocal crosses is extremely common in 

 a lesser degree. He has observed it even l)etween closely-re- 

 lated forms (as Matthiola annua and glabra) which many bota- 

 nists rank only as varieties. It is also a remarkable fact, that 

 hybrids raised from reciprocal crosses, though of course com- 

 pounded of the very same two species, fhe one species having 



