244 LAWS GOVERNING THE STERILITY [Chap. IX 



facility in making reciprocal cresses is extremely common in a 

 lesser degree. He has observed it even between closely related 

 forms (as Mattbiola 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 occa- 

 sionally 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 intermediate 

 character between their two parents, always closely resemble one of 

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

 parent-species, are with rare exceptions extremely sterile. So agarn 

 amongst hybrids which are usually intermediate in structure 

 between their parents, exceptional and abnormal individuals some- 

 times are bom, which closely resemble one of theii pure parents ; 

 and these hybrids are almost always utterly sterile, even when the 

 other hybrids raised from seed from the same capsule have a con- 

 siderable degree of fertility. These facts show how completely the 

 fertility of a hybrid may be independent 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 

 fertility under certain conditions in excess; that their fertility, 

 besides being eminently susceptible to favourable and unfavourable 

 conditions, is innately variable ; that it is by no means always 

 the same in degree in the first cross and in the hybrids produced 

 from this cross ; that the fertility of hybrids is not related to the 

 degree in which they resemble in external appearance either parent ; 

 and lastly, that the facility of making a first cross between any 

 two species is not always governed by their systematic affinity or 

 degree of resemblance to each other. This latter statement is 

 clearly proved by the difference in the result of reciprocal crosses 

 between the same two species, for, according as the one species 

 or the other is used as the father or the mother, there is generally 

 some difference, and occasionally the widest possible differeDOi 1 , 



