I. page 136.) Here again it is clear that though
he credits Agassiz with having called vivid attention to the Glacial
period, he had himself much earlier grasped the idea of periods of
refrigeration.

Putting aside the fact, which has only been made known to us since
Darwin's death, that he had anticipated Forbes, it is clear that he gave
the theory a generality of which the latter had no conception. This is
pointed out by Hooker in his classical paper "On the Distribution of
Arctic Plants" (1860). "The theory of a southern migration of northern
types being due to the cold epochs preceding and during the glacial,
originated, I believe, with the late Edward Forbes; the extended one,
of the trans-tropical migration, is Mr Darwin's." ("Linn. Trans."
XXIII. page 253. The attempt appears to have been made to claim for Heer
priority in what I may term for short the arctic-alpine theory (Scharff,
"European Animals", page 128). I find no suggestion of his having hit
upon it in his correspondence with Darwin or Hooker. Nor am I aware
of any reference to his having done so in his later publications. I
am indebted to his biographer, Professor Schroter, of Zurich, for an
examination of his earlier papers with an equally negative result.)
Assuming that local races have derived from a common ancestor, Hooker's
great paper placed the fact of the migration on an impregnable basis.
And, as he pointed out, Darwin has shown that "such an explanation meets
the difficulty of accounting for the restriction of so many American and
Asiatic arctic types to their own peculiar longitudinal zones, and for
what is a far greater difficulty, the representation of the same arctic
genera by most closely allied species in different longitudes."

The facts of botanical geography were vital to Darwin's argument. He
had to show that they admitted of explanation without assuming multiple
origins for species, which would be fatal to the theory of Descent. He
had therefore to strengthen and extend De Candolle's work as to means
of transport. He refused to supplement them by hypothetical geographical
changes for which there was no independent evidence: this was simply to
attempt to explain ignotum per ignotius. He found a real and, as it has
turned out, a far-reaching solution in climatic change due to cosmical
causes which compelled the migration of species as a condition of their
existence. The logical force of the argument consists in dispensing with
any v