Nor is it surprising that
the facility of effecting a first cross, the fertility of the hybrids
produced, and the capacity of being grafted together--though this
latter capacity evidently depends on widely different
circumstances--should all run, to a certain extent, parallel with the
systematic affinity of the forms which are subjected to experiment;
for systematic affinity attempts to express all kinds of resemblance
between all species.
First crosses between forms known to be varieties, or sufficiently
alike to be considered as varieties, and their mongrel offspring, are
very generally, but not quite universally, fertile. Nor is this nearly
general and perfect fertility surprising, when we remember how liable
we are to argue in a circle with respect to varieties in a state of
nature; and when we remember that the greater number of varieties have
been produced under domestication by the selection of mere external
differences, and not of differences in the reproductive system. In all
other respects, excluding fertility, there is a close general
resemblance between hybrids and mongrels. Finally, then, the facts
briefly given in this chapter do not seem to me opposed to, but even
rather to support the view, that there is no fundamental distinction
between species and varieties.
CHAPTER 9. ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day.
On the nature of extinct intermediate varieties; on their number.
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