Chap. ix. of first Crosses and of Hybrids. 243 



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 ^enus 

 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. 

 Many analogous facts could be given. 



No one has been able to point out what kind or what amounjj 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 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 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 : 

 ilirabilu jalapa can easily be fertilised by the pollen of M. longi- 

 flora, and the hybrids thus produced are sufficiently fertile ; but 

 ivolreuter tried more than two hundred times, during eight fol- 

 lowing years, to fertilise 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 



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