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Show 302 LAWS OF VARIATION. CHAP. XXIV. The epidermis on our hands is easily thickened, as every one knows, by hard work. In a district of Ceylon the sheep have "horny callosities that defend their knees, and which arise from " their habit of kneeling down to crop the short herbage, and '' this distinguishes the J affna flocks from those of other portions " of the island ; " but it is not stated whether this peculiarity is inherited.32 · The mucous membrane which lines the stomach is continuous with the external skin of the body ; therefore it is not surprising that its texture should be affected by the nature of the food consumed, but other and more interesting changes likewise follow. Hunter long ago observed that the muscular coat ofthe stomach of a gull (Larus tridactylus) which had been fed for a year chiefly on grain was thickened; and, according to Dr. Edmondston, a similar change periodically occurs in the Shetland Islands in the stomach of the Larus argentat·us, which in the spring frequents the corn-fields and feeds on the seed. The same careful observer has noticed a great change in the stomach of a raven which had been long fed on vegetable food. In the case of an owl (Strix grallaria) similarly treated, Menetries states that the form of the stomach was changed, the inner coat became leathery, and the liver increased in size. Whether these modifications in the digestive organs would in the course of generations become inherited is not' known. 33 The increased or diminished length of the intestines, which apparently results from changed diet, is a more remarkable case, because it is characteristic of certain animals in their domesticated condition, and therefore must be inherited. The complex absorbent system, the blood-vessels, nerves, and muscles, are necessarily all modified together with the intestines. According to Daubenton, the intestines of the domestic cat are one-third longer than those of the wild cat of Europe; and although this species is not the parent-stock of the domestic animal, yet, as Isidore Geoffroy has remarked, the several species 32 'Ceylon,' by Sir J. E. Tennent, 1859, vol. ii. p. 531. 33 For the foregoing statements, see Hunter's 'Essays and Observations,' 1861, vol. ii. p. 329; Dr. Edmondston, as quoted in Macgillivray's 'British Birds,' vol. v. p. 550; Menetries, as quoted in Bronn's ' Geschichtc der Natur,' B. ii. s. llO. CHAP, XXIV. CHANGED HABITS OF LIFE. 303 of cats are so closely allied that the comparison is probably a fair one. The increased length appears to be due to the domestic cat being less strictly carnivorous in its diet than any wild feline species ; I have seen a French kitten eating vegetables as readily as meat. According to Cuvier, the intestines of the domesticated pig exr-eed greatly in proportionate length those of the wild boar. In the tame and wild rabbit the change is of an opposite nature, and probably results from the nutritious food given to the tame rabbit.34 Changed Habits of Life, independently of the Use or Disuse of particular Organs.-This subject, as far as the mental powers of animals are concerned, so blends into instinct, on which I shall treat in a future work, that I will here only remind the reader of the many cases which occur under domestication, and which are familiar to every one-for instance the tameness of our animals-the pointing or retrieving of dogstheir not attacking the smaller animals kept by man-and so forth. How murh of these changes ought to be attributed to inherited habit, and how much to the selection of individuals which have varied in the desired manner, irrespectively of the special circumstances under which they have been kept, can seldom be told. We have aheady seen that animals may be habituated to a changed diet; but a few additional instances may here be given. In the Polynesian Islands and in China the dog is fed exclusively on vegetable matter, and the taste for this kind of food is to a certain extent inherited.35 Our sporting dogs will not touch the bones of game birds, whilst other dogs devour them with greediness. In some parts of the world sheep have been largely fed on fish. The domestic hog is fond of barley, the wild boar is said to disdain it; and the disdain is partially inherited, for some young wild pigs bred in captivity showed an aversion for this grain, whilst others of the same brood relished it. 36 One of my relations bred some young pigs from 34 These statements on the intestines are taken from I sidore Geoffroy St. Hilaire, 'Hist. Nat. Gen.,' tom. iii. pp. 427, 441. as Gilbert White, 'Nat. Hist. Sel-bourne,' 1825, vol. ii. p. 121. 36 Burdacb, 'Traite de Phys.,' tom. ii. p. 267, as quoted by Dr. P. Lucas, 'L'Hered. Nat.,' tom. j_ p. 388. |
