We observe from the earliest times a rhythmic play, as it were, of
opposite forces that tends, alternately, to build up and to break down
and mingle human races, but of the laws that underlie and govern these
forces we know little or nothing. On the one hand we see how man has
always and everywhere shown what the advocates of so-called racial
purity have called "a perverse predisposition to mismate" which has made
it exceedingly difficult to classify existing human varieties. On the
other hand we see throughout nature how a pronounced disparity between
varieties of the same species engenders an aversion from one another of
the different varieties which seems to arise, in men and animals alike,
through the instinct of sexual jealousy which is probably bound up with
the primary instinct of self-preservation. Those people who profess
belief in the inherent superiority of a particular race naturally look
upon the tendency towards race-blending as a perverse proclivity, while
those who think that all men are potentially equal regard it as a
wholesome instinct provided by nature to counteract the feebleness and
infertility which cause the dying-out of the race that becomes too
pure.