ngle or multiple centres of creation differs from
another though allied question, namely, whether all the individuals
of the same species are descended from a single pair, or single
hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With organic beings which never
intercross, if such exist, each species, must be descended from a
succession of modified varieties, that have supplanted each other,
but have never blended with other individuals or varieties of the same
species, so that, at each successive stage of modification, all the
individuals of the same form will be descended from a single parent.
But in the great majority of cases, namely, with all organisms which
habitually unite for each birth, or which occasionally intercross, the
individuals of the same species inhabiting the same area will be kept
nearly uniform by intercrossing; so that many individuals will go on
simultaneously changing, and the whole amount of modification at each
stage will not be due to descent from a single parent. To illustrate
what I mean: our English race-horses differ from the horses of every
other breed; but they do not owe their difference and superiority to
descent from any single pair, but to continued care in the selecting and
training of many individuals during each generation.

Before discussing the three classes of facts, which I have selected as
presenting the greatest amount of difficulty on the theory of "single
centres of creation," I must say a few words on the means of dispersal.

MEANS OF DISPERSAL.

Sir C. Lyell and other authors have ably treated this subject. I can
give here only the briefest abstract of the more important facts. Change
of climate must have had a powerful influence on migration. A region now
impassable to certain organisms from the nature of its climate, might
have been a high road for migration, when the climate was different. I
shall, however, presently have to discuss this branch of the subject
in some detail. Changes of level in the land must also have been highly
influential: a narrow isthmus now separates two marine faunas; submerge
it, or let it formerly have been submerged, and the two faunas will now
blend together, or may formerly have blended. Where the sea now extends,
land may at a former period have connected islands or possibly even
continents together, and thus have allowed terrestrial productions to
pass from one to the other. No geologi