he early death of the embryo. The sterility of
hybrids, which have their reproductive systems imperfect, and which have
had this system and 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 from it, 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 {278} 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.

* * * * *

{279}

CHAPTER IX.

ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day--On the
natur