[35] Ibid. pp. 157-8.
[36] Life and Letters, vol. iii. p. 161.
[37] Page 81. The three forms of isolation mentioned are, "from haunting different stations, from breeding at slightly different seasons, or from the individuals of each variety preferring to pair together."
[38] Life and Letters, vol. iii. p. 159.
[39] Life and Letters, vol. iii. p. 155.
[40] Variation, &c., vol. ii. p. 262.
[41] Life and Letters, vol. iii. p. 161.
[42] Die Darwin'sche Theorie und das Migrationsgesetz (1868): Ueber den Einfluss der geographischen Isolirung, &c. (1870).
[43] For instance, speaking of common, or continuous areas, he says:—"In this case a constant variety, or new species, cannot be produced, because the free crossing of a new variety with the old unaltered stock will always cause it to revert to the original type; in other words, will destroy the new form. The formation of a real variety, which Darwin, as we know, regards as the commencement of a new species, will only succeed when a few individuals, having crossed the barrier of their habitat, are able to separate themselves for a long time from the old stock." And the last sentence, given as a summary of his whole doctrine, is—"The geographical isolation of the form, a necessary consequence of migration, is the cause of its typical character."
[44] Ueber den Einfluss der Isolirung auf die Artbildung (1872).