the unknown causes which produce them were to act
more uniformly, they would probably become common to all the individuals
of the species. We may hope hereafter to understand something about the
causes of such occasional modifications, especially through the study of
monstrosities; hence, the labours of experimentalists, such as those of
M. Camille Dareste, are full of promise for the future. In general we
can only say that the cause of each slight variation and of each
monstrosity lies much more in the constitution of the organism than in
the nature of the surrounding conditions; though new and changed
conditions certainly play an important part in exciting organic changes
of many kinds.

Through the means just specified, aided perhaps by others as yet
undiscovered, man has been raised to his present state. But since he
attained to the rank of manhood, he has diverged into distinct races,
or, as they may be more fitly called, subspecies. Some of these, such as
the negro and European, are so distinct that, if specimens had been
brought to a naturalist without any further information, they would
undoubtedly have been considered by him as good and true species.
Nevertheless, all the races agree in so many unimportant details of
structure and in so many mental peculiarities, that these can be
accounted for only by inheritance from a common progenitor; and a
progenitor thus characterized would probably deserve to rank as man.

It must not be supposed that the divergence of each race from the other
races, and of all from a common stock, can be traced back to any one
pair of progenitors. On the contrary, at every stage in the process of
modification, all the individuals which were in any way best fitted for
their conditions of life, though in different degrees, would have
survived in greater numbers than the less well-fitted. The process would
have been like that followed by man, when he does not intentionally
select particular individuals, but breeds from all the superior
individuals and neglects all the inferior individuals. He thus slowly
but surely modifies his stock and unconsciously forms a new strain. So
with respect to modifications acquired independently of selection, and
due to variations arising from the nature of the organism and the
action of the surrounding conditions, or from changed habits of life, no
single pair will have been modified in a much greater degree than the
other pairs which inhabit the same cou