[289] [This law has been beautifully applied by Herbert Spencer in order to explain why, with an unlimited supply of food, an organism does not indefinitely increase in size. “Principles of Biology,” vol. i. p. 121-126. R.M.] [290] [Eng. ed. This idea, formerly expressed by me, occurs also in Lange (“Geschichte des Materialismus,” ii. 265), and is there exemplified in a very beautiful manner by illustrations from modern chemistry. Lange compares what I have termed above the “physical constitution” of the organism to the chemical constitution of one of those organic acids which by substitution of single elements may become transformed into more complicated acids, but which, as it were, always undergo “further development” in only one determined and narrowly restricted course. Here, as with the organism, the number of possible variations is very great, but is nevertheless limited, since “what can or cannot arise is determined beforehand by certain hypothetical properties of the molecule.”] [291] “Origin of Species.” 4th German ed., p. 19; 5th English ed., p. 6. [292] [Mr. A. R. Wallace, in his article last referred to, quotes some most valuable measurements of mammals and birds, showing the amount of variation of the different parts. These observations were published by J. A. Allen, in a memoir “On the Mammals and Winter Birds of East Florida,” &c. (Bulletin of the Museum of Comparative Zoology at Harvard College, Cambridge, Mass., vol. ii. No. 3.) R.M.] [293] [See note 142, p. 310. R.M.] [294] “Die Darwin’sche Theorie,” Dorpat, 1875. [295] [A certain number of instances of mimicry are known to occur between species both of which are apparently nauseous. A most able discussion of this difficult problem is given by Fritz Müller, in the case of the two butterflies _Ituna Ilione_ and _Thyridia Megisto_, in a paper published in _Kosmos_, May, 1879 (p. 100). The author shows by mathematical reasoning that such resemblances between protected species can be accounted for by natural selection if we suppose that young birds and other insect persecutors have to learn by experience which species are distasteful and which can be safely devoured. See also Proc. Ent. Soc. 1879, pp. xx-xxix. R.M.] [296] See Haeckel’s “Generelle Morphologie,” ii. 107. [297] “Über die Berechtigung der Darwin’schen Theorie,” Leipzig, 1868. [298] “Populäre wissenschaftl. Vorträge,” vol. ii., Brunswick, 1871, p. 208. [299] “Das Unbewusste vom Standpunkte der Physiologie u. Descendenztheorie,” Berlin, 1872, p. 89. The second edition appeared in 1877, in Von Hartmann’s own name.