The Descent of Man

e produced at the rarest intervals. But there is
even a higher grade of sterility than this. Both
Gartner and Kolreuter have proved that in genera
of plants, including many species, a series
can be formed from species which, when crossed,
yield fewer and fewer seeds, to species
which never produce a single seed, but yet are
affected by the pollen of the other species, as
shewn by the swelling of the germen. It is here
manifestly impossible to select the more sterile
individuals, which have already ceased to yield
seeds; so that the acme of sterility, when the
germen alone is affected, cannot have been gained
through selection. This acme, and no doubt
the other grades of sterility, are the incidental
results of certain unknown differences in the
constitution of the reproductive system of the
species which are crossed.), that domestication
tends to eliminate the sterility which is so general
a result of the crossing of species in a state
of nature. From these several considerations, it
may be justly urged that the perfect fertility of