308 ORIGIN OF SPECIES 



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 com- 

 pletely 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 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 common in a lesser degree. 

 He has observed it even between closely related forms (as 

 Matthiola annua and glabra) which many botanists rank 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, though they rarely 

 differ in external characters, yet generally differ in fertility 

 in a small, and occasionally in a high degree. 



Several other singular rules could be given from Gartner: 

 for instance, some species have a remarkable power of cross- 

 ing with other species; other species of the same genus have 

 a remarkable power of impressing their likeness on their 

 hybrid offspring; but these two powers do not at all neces- 

 sarily go together. There are certain hybrids which, instead 



