We thus see that not only do those parts of the skin which bear white hair differ in a remarkable manner from those bearing hair of any other colour, but that some great constitutional difference must be correlated with the colour of the hair; for in the above-mentioned cases, vegetable poisons caused fever, swelling of the head, as well as other symptoms, and even death, to all the white, or white-spotted animals.
CHAPTER 2.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'affinite de soi pour soi, which has been discussed and illustrated by his son, Isidore, with respect to monsters in the animal kingdom (26/1. 'Hist. des Anomalies' 1832 tome 1 pages 22, 537-556; tome 3 page 462.), 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 (26/2. 'Comptes Rendus' 1855 pages 855, 1039.), 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 (26/3. 'Catalogue of the Teratological Series in the Museum of the R. Coll. of Surgeons' 1872 page 16.): "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 (26/4. 'Archives de Zoolog. Exper.' January 1874 page 78.) 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 a un meme oeuf; qu'ils s'unissent en meme temps qu'ils se forment, et que la soudure ne se produit que pendant la premiere periode de la vie embryonnaire, celle ou les organes ne sont encore constitues que par des blastemes homogenes."
By whatever means the abnormal fusion of homologous parts is effected, such cases throw light on the frequent presence of organs which are double during an embryonic period (and throughout life in other and lower members of the same class) but which afterwards unite by a normal process into a single medial organ. In the vegetable kingdom Moquin-Tandon (26/5. 'Teratologie Veg.' 1841 livre 3.) gives a long list of cases, showing how frequently homologous parts, such as leaves, petals, stamens, and pistils, flowers, and aggregates of homologous parts, such as buds, as well as fruit, become blended, both normally and abnormally, with perfect symmetry into one another.
THE VARIABILITY OF MULTIPLE AND HOMOLOGOUS PARTS.
Isidore Geoffroy (26/6. 'Hist. des Anomalies' tome 3 pages 4, 5, 6.) insists that, when any part or organ is repeated many times in the same animal, it is particularly liable to vary both in number and structure. With respect to number, the proposition may, I think, be considered as fully established; but the evidence is chiefly derived from organic beings living under their natural conditions, with which we are not here concerned. Whenever such parts as the vertebrae or teeth, the rays in the fins of fishes, or the feathers in the tails of birds, or petals, stamens, pistils, or seeds, are very numerous, the number is generally variable. With respect to the structure of multiple parts, the evidence of variability is not so decisive; but the fact, as far as it may be trusted, probably depends on multiple parts being of less physiological importance than single parts; consequently their structure has been less rigorously guarded by natural selection.