256 : 
HYBRIDISM. 
Chap. VIII. 
with the pollen of either pure parent^ a single fertile seed: 
but in some of these cases a first trace of fertility may 
be detected, by the pollen of one of the pure parent- 
species causing the fiower of the hybrid to wither 
earlier than it otherwise would have done; and the 
early withering of the flower is well known to be a sign 
of incipient fertilisation. From this extreme degree 
of sterility we have self-fertilised hybrids producing a 
greater and greater number of seeds up to perfect fertility. 
Hybrids from two species which are very diflScult to 
cross, and which rarely produce any offspring, are gene¬ 
rally very sterile; but the parallelism between the diffi¬ 
culty of making a first cross, and the sterility of the 
hybrids thus produced—^two classes of facts which are 
generally confounded together—is by no means strict. 
There are many cases, in which two pure species can be 
united with unusual facility, and produce numerous 
hybrid-offspring, yet these hybrids are remarkably 
sterile. On the other hand, there are species which 
can be crossed very rarely, or with extreme difficulty, 
but the hybrids, when at last produced, are very fertile. 
Even within the limits of the same genus, for instance 
in Dianthus, these two opposite cases occur. 
The fertility, both of first crosses and of hybrids, is 
more easily affected by unfavourable conditions, than 
is the fertility of pure species. But the degree of 
fertility is likewise innately variable ; for it is not always 
the same when the same two species are crossed under 
the same circumstances, but depends in part upon the 
constitution of the individuals which happen to have 
been chosen for the experiment. So it is with hybrids, 
for their degree of fertility is often found to differ 
greatly in the several individuals raised from seed out 
of the same capsule and exposed to exactly the same 
conditions. 
