Air, Water, and Places'.[3]

In the modern world, too, no serious doubt was cast on the specific
unity of mankind, handed down from antiquity, until Linnaeus and Buffon
had refined upon the biological notions of genus and species (for both
of which there is only one word in Greek), and had defined species by
the criterion of fertility. Now not only the great explorers, but every
ship's captain, knew by this time that white men, at all events, would
form fertile unions with all known kinds of humanity. But in the
eighteenth century it became known also, and in the same empirical way,
that the fertility of unions between white men and black was imperfect;
and as this was the only human cross for which there was any large
quantity of evidence, the impression grew that the zoological distance
between these races was greater than had been supposed. On the other
hand, eighteenth-century formulators of the 'Rights of Man' challenged
reconsideration of the current practice of negro slavery; and the upshot
was a controversy. Abolitionists contended that the 'black brother' was
indeed a blood brother, and entitled to the 'Rights of Man'; their
opponents replied that the negro, being (as they held) of another
species, might justly be treated in all respects as one of white man's
domestic animals, and be his property as well as his drudge. At the turn
of the century, the adherence of Cuvier gave prestige to Polygenesis on
its scientific side: and it took all the reasonableness of Prichard in
the next generation to turn the tide even in England. But the issue of
the American Civil War, to which reference has already been made,
coincided so closely in time with the work of Darwin and Lyell on the
real meaning of species and on the antiquity of man, that the
controversy was closed without bitterness. The new phase of Polygenism
which seems now to be opening, with successive discoveries of the
quaternary stratification of races, and Keith's analysis of the family