[37] Mr. Herbert Spencer (‘Principles of Biology,’ 1864, vol. i. pp. 452, 468) takes a different view; and in one place remarks: “We have seen reason to think that, as fast as essential faculties multiply, and as fast as the number of organs that co-operate in any given function increases, indirect equilibration through natural selection becomes less and less capable of producing specific adaptations; and remains fully capable only of maintaining the general fitness of constitution to conditions.” This view that natural selection can do little in modifying the higher animals surprises me, seeing that man’s selection has undoubtedly effected much with our domesticated quadrupeds and birds.
[38] Dr. Prosper Lucas apparently disbelieves in any such connection; ‘L’Héréd. Nat.,’ tom. ii. pp. 88-94.
[39] ‘British Medical Journal,’ 1862, p. 433.
[40] Boudin, ‘Géograph. Médicale,’ tom. i. p. 406.
[41] This fact and the following cases, when not stated to the contrary, are taken from a very curious paper by Prof. Heusinger, in ‘Wochenschrift fur Heilkunde,’ May, 1846, s. 277. Settegast (‘Die Thierzucht,’ 1868, p. 39) says that white or white-spotted sheep suffer like pigs, or even die from eating buckwheat; whilst black or dark-woolled individuals are not in the least affected.
[42] Mr. Mogford, in the ‘Veterinarian,’ quoted in ‘The Field,’ Jan. 22nd, 1861, p. 545.
[43] ‘Edinburgh Veterinary Journal,’ Oct. 1860, p. 347.
CHAPTER XXVI.
LAWS OF VARIATION, continued.—SUMMARY.
THE FUSION OF HOMOLOGOUS PARTS—THE VARIABILITY OF MULTIPLE AND HOMOLOGOUS PARTS—COMPENSATION OF GROWTH—MECHANICAL PRESSURE—RELATIVE POSITION OF FLOWERS WITH RESPECT TO THE AXIS, AND OF SEEDS IN THE OVARY, AS INDUCING VARIATION—ANALOGOUS OR PARALLEL VARIETIES—SUMMARY OF THE THREE LAST CHAPTERS.
The Fusion of Homologous Parts.—Geoffroy Saint-Hilaire formerly propounded what he called la loi de l’affinité de soi pour soi, which has been discussed and illustrated by his son, Isidore, with respect to monsters in the animal kingdom,[[1]] and by Moquin-Tandon, with respect to monstrous plants. This law seems to imply that homologous parts actually attract one another and then unite. No doubt there are many wonderful cases, in which such parts become intimately fused together. This is perhaps best seen in monsters with two heads, which are united, summit to summit, or face to face, or Janus-like, back to back, or obliquely side to side. In one instance of two heads united almost face to face, but a little obliquely, four ears were developed, and on one side a perfect face, which was manifestly formed by the fusion of two half-faces. Whenever two bodies or two heads are united, each bone, muscle, vessel, and nerve on the line of junction appears as if it had sought out its fellow, and had become completely fused with it. Lereboullet,[[2]] who carefully studied the development of double monsters in fishes, observed in fifteen instances the steps by which two heads gradually became united into one. In all such cases it is now thought by the greater number of capable judges that the homologous parts do not attract each other, but that in the words of Mr. Lowne:[[3]] “As union takes place before the differentiation of distinct organs occurs, these are formed in continuity with each other.” He adds that organs already differentiated probably in no case become united to homologous ones. M. Dareste does not speak[[4]] quite decisively against the law of soi pour soi, but concludes by saying, “On se rend parfaitement compte de la formation des monstres, si l’on admet que les embryons qui se soudent appartiennent à un même œuf; qu’ils s’unissent en même temps qu’ils se forment, et que la soudure ne se produit que pendant la première période de la vie embryonnaire, celle ou les organes ne sont encore constitués que par des blastèmes homogènes.”