[AW] "In every case there are two factors, namely, the nature of the organism and the nature of the conditions. The former seems to be much the more important; for nearly similar variations sometimes arise under, as far as we can judge dissimilar conditions; and, on the other hand, dissimilar variations arise under conditions which appear to be nearly uniform" ("Origin of Species," p. 6). [AX] See "Evolution without Natural Selection," by Charles Dixon. This author's facts are valuable; his theories are ill digested. [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. CHAPTER V. HEREDITY AND THE ORIGIN OF VARIATIONS. The law of heredity, I have said above, may be regarded as that of persistence exemplified in a series of organic generations. Variation results--it is clear that it must result--from some kind of differentiating influence. Such statements as these, however, though they are true enough, do not help us much in understanding either heredity or variation.