Three accounts have been published in Eastern Prussia, of white and white- spotted horses being greatly injured by eating mildewed and honeydewed vetches; every spot of skin bearing white hairs becoming inflamed and gangrenous. The Rev. J. Rodwell informs me that his father turned out about fifteen cart-horses into a field of tares which in parts swarmed with black aphides, and which no doubt were honeydewed, and probably mildewed; the horses, with two exceptions, were chestnuts and bays with white marks on their faces and pasterns, and the white parts alone swelled and became angry scabs. The two bay horses with no white marks entirely escaped all injury. In Guernsey, when horses eat fool's parsley (Aethusa cynapium) they are sometimes violently purged; and this plant "has a peculiar effect on the nose and lips, causing deep cracks and ulcers, particularly on horses with white muzzles." (25/42. Mr. Mogford in the 'Veterinarian' quoted in 'The Field' January 22, 1861 page 545.) With cattle, independently of the action of any poison, cases have been published by Youatt and Erdt of cutaneous diseases with much constitutional disturbance (in one instance after exposure to a hot sun) affecting every single point which bore a white hair, but completely passing over other parts of the body. Similar cases have been observed with horses. (25/43. 'Edinburgh Veterinary Journal' October 1860 page 347.)
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.