With respect to varieties which have originated in a state of nature, it is
almost hopeless to expect to prove by direct evidence that they have been
rendered mutually sterile; for if even a trace of sterility could be
detected, such varieties would at once be raised by almost every naturalist
to the rank of distinct species. If, for instance, Gaertner's statement
were fully confirmed, that the blue and red-flowered forms of the pimpernel
(_Anagallis arvensis_) are sterile when crossed, I presume that all the
botanists who now maintain on various grounds that these two forms are
merely fleeting varieties, would at once admit that they were specifically
distinct.

The real difficulty in our present subject is not, as it appears to me, why
domestic varieties have not become mutually infertile when crossed, but why
this has so generally occurred with natural varieties as soon as they have
been modified in a sufficient and permanent degree to take rank as species.
We are far from precisely knowing the cause; nor is this surprising, seeing
how profoundly ignorant we are in regard to the normal and abnormal action
of the reproductive system. But we can see that species, owing to their
struggle for life with numerous competitors, must have been exposed to more
uniform conditions during long periods of time, than have been domestic
varieties; and this may well make a wide difference in the result. For we
know how commonly wild animals and plants, when taken from their natural
conditions and subjected to captivity, are rendered sterile; and the
reproductive functions of organic beings which have always lived and been
slowly modified under natural conditions would probably in like manner be
eminently sensitive to the influence of an unnatural cross. Domesticated
productions, on the other hand, which, as shown by the mere fact of their
domestication, were not originally highly sensitive to changes in their
conditions of life, and which can now generally resist {191} with
undiminished fertility repeated changes of conditions, might be expected to
produce varieties, which would be little liable to have their reproductive
powers injuriously affected by the act of crossing with other varieties
which had originated in a like manner.

Certain naturalists have recently laid too great stress, as it appears to
me, on the difference in fertility between varieties and species when
crossed. Some allied species of trees cannot be grafted on