Darwin and Modern Science [211]
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 violent 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
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 violent 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