20 THK ORIGIN OF SPECIES 



minata, wliich is not a particularly distinct species, obsti- 

 nately 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 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 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 independent of 

 their systematic affinity, that is of any difference in their 

 structure or constitution, excepting in their reproductive 

 systems. The diversity of the result in reciprocal crosses 

 between the same two species was long ago observed by 

 Kolreuter. To give an instance: Mirabilis jalapa can 

 easily be fertilized by the pollen of M. longiflora, and 

 the hybrids thus produced are sufficiently fertile; but 

 Kolreuter tried more than two hundred times, during 

 eight following years, to fertilize reciprocally M. long- 



