Chap. VIII. 
LAWS OF STEEILITY. 
259 
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 
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 again amongst hybrids which are usually 
intermediate in structure between their parents, ex¬ 
ceptional and abnormal 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 fertility. These 
facts show how completely fertility in the hybrid is 
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 
