C'HAP. IX.J OF FIRST CROSSES AND OF HYBRIDS. 227 



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 

 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 amount 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 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 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 fertilised 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 fertilise reciprocally M. longiflora witk 

 the pollen of M. jalapa, and utterly failed. Several other equallj 



