pendent creation. This view of the
relation of species in one region to those in another, does not differ much
(by substituting the word variety for species) from that lately advanced in
an ingenious paper by Mr. Wallace, in which he concludes, that "every
species has come into existence coincident both in space and time with a
pre-existing closely allied species." And I now know from correspondence,
that this coincidence he attributes to generation with modification.

The previous remarks on "single and multiple centres of creation" do not
directly bear on another allied question,--namely whether all the
individuals of the same species have descended from a single pair, or
single hermaphrodite, or whether, as some authors suppose, from many
individuals simultaneously created. With those organic beings which never
intercross (if such exist), the species, on my theory, must have descended
from a succession of improved varieties, which will never have blended with
other individuals or varieties, but will have supplanted each other; so
that, at each {356} successive stage of modification and improvement, all
the individuals of each variety will have descended from a single parent.
But in the majority of cases, namely, with all organisms which habitually
unite for each birth, or which often intercross, I believe that during the
slow process of modification the individuals of the species will have been
kept nearly uniform by intercrossing; so that many individuals will have
gone on simultaneously changing, and the whole amount of modification will
not have been due, at each stage, to descent from a single parent. To
illustrate what I mean: our English racehorses differ slightly 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
selecting and training many individuals during many generations.

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 when its climate was different may have been a high
road for migration, but now be impassable;