[AY]Nature, vol. xlii. p. 136.
[AZ]We may here note, in passing, the fact that the changes of life-forms in a succession of beds points in nine cases out of ten rather to substitution through migration than to transmutation. Still, there are notable cases of transmutation, as in the fresh-water Planorbes of Steinhem, in Wittenberg (described, after Hilgendorf, by O. Schmidt, "The Doctrine of Descent," p. 96).
[BA]I would ask historians whether there have not been, in English history, good times of free and beneficial divergence exemplified in diverse intellectual activity, hard times of rigorous elimination, and intermediate times of placid, somewhat humdrum conservatism.
[BB]Two more technical examples may be noticed in a note. (1) Professor Haeckel has recently (Challenger Reports, vol. xxviii.) shown that the Siphonophora include two groups, closely resembling each other, but of different ancestry: (a) The Disconanthæ, traceable to trachomedusoid ancestors; (b) the Siphonanthæ, traceable to anthomedusoid ancestors like Sarsia. (2) M. Paul Pelseneer has been led to the conclusion that the pteropod molluscs also include two groups resembling each other, but of different ancestry: (a) The Thecosomes, traceable to tornatellid ancestors; (b) the Gymnosomes, traceable to aphysiid ancestors. In each case, the ancestral sea-slug has been modified for a free-swimming life.
[BC]For evidence in copious abundance, see Nicholson's "Manual of Palæontology," new edition, vol. i.: "Vertebrata," by R. Lydekker.
[BD]"Animals and Plants under Domestication," vol ii. p. 239.
[BE]Or in certain "physiological units" (Herbert Spencer), or "plastidules" (Haeckel), which may be regarded as organic molecules exhibiting their special properties under vital conditions.
[BF]Nature, vol. xxxix. p. 486.
[BG]Darwin, "Animals and Plants under Domestication," 2nd edit., vol. ii. chap. xxvii., from which the following description and quotations are taken.
[BH]For an excellent account of the genesis and growth of the modern views of heredity, see Mr. J. Arthur Thomson's paper on "The History and Theory of Heredity:" Proceedings of the Royal Society of Edinburgh, 1889.