258 HYBRIDISM. Chap. VIII. 



By a reciprocal cross between two species, I mean 

 the case, for instance, of a stallion-horse being first 

 crossed with a female-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 that 

 the capacity in any two species to cross is often com- 

 pletely independent of their systematic affinity, or of 

 any recognisable difference in their whole organisation. 

 On the other hand, these cases clearly show that the 

 capacity for crossing is connected with constitutional 

 differences imperceptible by us, and confined to the 

 reproductive system. This difference in the result of 

 reciprocal crosses between the same two species was 

 long ago observed by Kolreuter. To give an instance : 

 Mirabilis jalappa 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 with the pollen of M. jalappa, 

 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 reci- 

 procal crosses is extremely common in a lesser degree. 

 He has observed it even between forms so closely related 

 (as Matthiola annua and glabra) that many botanists 

 rank them only as varieties. It is also a remarkable 

 fact, that hybrids raised from reciprocal crosses, though 

 of course compounded of the very same two species, 

 the one species having first been used as the father and 

 then as the mother, generally differ in fertility in a 

 small, and occasionally in a high degree. 



Several other singular rules could be given from 



