amount of variation is
known; or could be predicted with due time and changes of condition
granted. It was then admitted that although the production of new races,
undistinguishable from true species, is probable, we must look to the
relations in the past and present geographical distribution of the
infinitely numerous beings, by which we are surrounded--to their
affinities and to their structure--for any direct evidence.

In the third chapter the inheritable variations in the mental phenomena
of domestic and of wild organic beings were considered. It was shown
that we are not concerned in this work with the first origin of the
leading mental qualities; but that tastes, passions, dispositions,
consensual movements, and habits all became, either congenitally or
during mature life, modified and were inherited. Several of these
modified habits were found to correspond in every essential character
with true instincts, and they were found to follow the same laws.
Instincts and dispositions &c. are fully as important to the
preservation and increase of a species as its corporeal structure; and
therefore the natural means of selection would act on and modify them
equally with corporeal structures. This being granted, as well as the
proposition that mental phenomena are variable, and that the
modifications are inheritable, the possibility of the several most
complicated instincts being slowly acquired was considered, and it was
shown from the very imperfect series in the instincts of the animals now
existing, that we are not justified in _prima facie_ rejecting a theory
of the common descent of allied organisms from the difficulty of
imagining the transitional stages in the various now most complicated
and wonderful instincts. We were thus led on to consider the same
question with respect both to highly complicated organs, and to the
aggregate of several such organs, that is individual organic beings; and
it was shown, by the same method of taking the existing most imperfect
series, that we ought not at once to reject the theory, because we
cannot trace the transitional stages in such organs, or conjecture the
transitional habits of such individual species.

In the Second Part{511} the direct evidence of allied forms having
descended from the same stock was discussed. It was shown that this
theory requires a long series of intermediate forms between the species
and groups in the same classes--forms not directly intermediate be