248 THE ORIGIN OF SPECIES 



highly important, for they prove that the capacity in any two 

 species to cross is often completely independent of their system- 

 atic affinity, that is, of any difference in their structure or consti- 

 tution, 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 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 oc- 

 casionally in a high, degree. 



Several other singular rules could be given from Gartner: for 

 instance, some species have a remarkable power of crossing 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 necessarily go together. There are 

 certain hybrids which, instead of having, as is usual, an inter- 

 mediate character between their two parents, always closely re- 

 semble one of them; and such hybrids, though externally so like 

 one of their pure parent-species, are with rare exceptions ex- 

 tremely sterile. So again among hybrids which are usually inter- 

 mediate in structure between their parents, exceptional and ab- 

 normal individuals sometimes are born, which closely resemble 

 one of their pure parents; and these hybrids are almost always 

 utterly sterile, even when the other hybrids raised from seed from 

 the same capsule have a considerable degree of fertihty. These 

 facts show how completely the fertility of a hybrid may be in- 

 dependent of its external resemblance to either pure parent. 



Considering the several rules now given, which govern the 

 fertility of first crosses and of hybrids, we see that when forms, 

 which must be considered as good and distinct species, are united, 

 their fertility graduates from zero to perfect fertility, or even to 



