iolent assumption, and in showing that the principle of descent is
adequate to explain the ascertained facts.

It does not, I think, detract from the merit of Darwin's conclusions
that the tendency of modern research has been to show that the effects
of the Glacial period were less simple, more localised and less
general than he perhaps supposed. He admitted that "equatorial
refrigeration... must have been small." ("More Letters", I. page 177.) It
may prove possible to dispense with it altogether. One cannot but regret
that as he wrote to Bates:--"the sketch in the 'Origin' gives a very
meagre account of my fuller MS. essay on this subject." (Loc. cit.)
Wallace fully accepted "the effect of the Glacial epoch in bringing
about the present distribution of Alpine and Arctic plants in the
NORTHERN HEMISPHERE," but rejected "the lowering of the temperature of
the tropical regions during the Glacial period" in order to account for
their presence in the SOUTHERN hemisphere. ("More Letters", II. page 25
(footnote 1).) The divergence however does not lie very deep. Wallace
attaches more importance to ordinary means of transport. "If plants can
pass in considerable numbers and variety over wide seas and oceans, it
must be yet more easy for them to traverse continuous areas of land,
wherever mountain-chains offer suitable stations." ("Island Life" (2nd
edition), London, 1895, page 512.) And he argues that such periodical
changes of climate, of which the Glacial period may be taken as a type,
would facilitate if not stimulate the process. (Loc. cit. page 518.)

It is interesting to remark that Darwin drew from the facts of plant
distribution one of his most ingenious arguments in support of this
theory. (See "More Letters", I. page 424.) He tells us, "I was led to
anticipate that the species of the larger genera in each country would
oftener present varieties, than the species of the smaller genera."
("Origin", page 44.) He argues "where, if we may use the expression, the
manufactory of species has been active, we ought generally to find the
manufactory still in action." (Ibid. page 45.) This proved to be the
case. But the labour imposed upon him in the study was immense. He
tabulated local floras "belting the whole northern hemisphere" ("More
Letters", I. page 107.), besides voluminous works such as De Candolle's
"Prodromus". The results scarcely fill a couple of pages. This is a good
illustration of the enormous pains which he to