ry slow
changes are in more danger of being accidentally diverted or
obliterated by crossing, and that the first stages of an incipient
change may be too unimportant to be permanent.

Many writers on the subject of the Unity of Man assume that any marked
variety must require a long time for its production. Our experience in
the case of the domestic animals teaches the reverse of this view; a
very important point too often overlooked.

3. The duration or permanence of varieties is very different. Some
return at once to the normal type when the causes of change are
removed. Others perpetuate themselves nearly as invariably as species,
and are named races. It is these races only that we are likely to
mistake for true species, since here we have that permanent
reproduction which is one of the characteristics of the species. The
race, however, wants the other characteristics of species as above
stated; and it differs essentially in having branched from a primitive
species, and in not having an independent origin. It is quite evident
that in the absence of historical evidence we must be very likely to
err by supposing races to have really originated in distinct
"primordial forms." Such error is especially likely to arise if we
overlook the fact of the sudden origination of such races, and their
great permanency if kept distinct. There are two facts which deserve
especial notice, as removing some of the difficulty in such cases. One
is that well-marked races usually originate only in domesticated
animals, or in wild animals which, owing to accidental circumstances,
are placed in abnormal circumstances. Another is, that there always
remains a tendency to return, in favorable circumstances, to the
original type. This tendency to reversion is much underrated by Darwin
and his followers; yet they constantly recur to it as a means of
proving possible derivation, and their writings abound in examples of
it. Perhaps the most remarkable of these reversions are those which
occur when varieties destitute of all the markings of the original
stock are crossed and reproduce those markings, which Darwin shows to
occur in pigeons and domestic fowls. The domesticated races usually
require a certain amount of care to preserve them in a state of
purity, both on this account and on account of the readiness with
which they intermix with other varieties of the same species. Many
very interesting facts in illustration of these points might be
ad