OF FIRST CROSSES AND OF HYBRIDS. 209 



tinct 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, ol> ( 

 stinately failed to fertilize, or to be fertilized by, no less i 

 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 suf- 

 ficient to prevent two species crossing. It can be shown 

 that plants most widely different in habit and general ap- 

 pearance, 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 difference in the facility of mak- 

 ing 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. 

 longiflora with the pollen of M. 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 commoa ia a lesser 



