their whole organisation disturbed by being compounded of two distinct
species, seems closely allied to that sterility which so frequently
affects pure species, when their natural conditions of life have been
disturbed. This view is supported by a parallelism of another
kind; namely, that the crossing of forms only slightly different is
favourable to the vigour and fertility of their offspring; and that
slight changes in the conditions of life are apparently favourable to
the vigour and fertility of all organic beings. It is not surprising
that the degree of difficulty in uniting two species, and the degree of
sterility of their hybrid-offspring should generally correspond, though
due to distinct causes; for both depend on the amount of difference of
some kind between the species which are crossed. 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 IX.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
211