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Show 262 CAUSES OF VARIABILITY. CHAP. XXII. same general effect. Mr. Salter 26 remarlrs, "Every ~n.e knows " that the chief difficulty is in breaking through the ongmal form " and colour of the species, and every one will be on the look-out "for any natural sport, either from seed or branch; that being "once obtained, however trifling the change may be, the result " depends upon himself." M. de Jonghe, who has had s.o much success in raising new varieties of pears and strawbernes,27 remarks with respect to the former, " There is another principle, "namely, that the more a type has entered into a state of variation, " the greater is its tendency to continue doing so ; and the more " it has varied from the m·iginal type, the more it is disposed to "vary still farther." We have, indeed, already discussed this latter point when treating of the power which man possesses, through selection, of continually augmenting in the same direction each modification; for this power depends on continued variability of the same general kind. The most celebrated horticulturist in France, namely, Vilmorin/8 even maintains that, when any particular variation is desired, the first step is to get the plant to vary in any manner whatever, and to go on selecting the most variable individuals, even though they vary in the wrong direction ; for the fixed character of the species being once broken, the desired variation will sooner or later appear. As nearly all our animals were domesticated at an extremely remote epoch, we cannot, of course, say whether they varied quickly or slowly when first subjected to new conditions. But Dr. Bachman 29 states that he has seen turkeys raised from the eggs of the wild species lose their metallic tints and become spotted with white in the third generation. Mr. Yarre11 many years ago informed me that the wild ducks bred on the ponds in St. James's Park, which had never been crossed, as it is believed, with domestic ducks, lost their true plumage after a few generations. An excellent observer/0 who has often reared birds from the eggs of the wild duck, and who took precautions 26 'The Chrysanthemum, its History, &r.,' 1865, p. 3. Z7 'Gardener's Chron.,' 1855, p. 54; 'Journal of Horticulture,' May 9, 1865, p. 363. 28 Quoted by Verlot, 'Des Varietes,' &c., 1865, p. 28. 29 ' Examination of the Characteristics of Genera and Species :' Charleston, 1855, p. 14. so Mr. Hewitt, 'Journal of Hort.,' 1863, p. 39. CHAP. XXII. CAUSES OF VARIABILITY. 263 that there should be no crossing with domestic breeds, has given, as previously stated, fu11 details on the changes which they gradually undergo. He found that he could not breed these wild ducks true for more than five or six generations, " as they then " proved so much less beautiful. The white collar round the neck "of the mallard became· much broader and more irregular, and " white feathers appeared in the ducklings' wings." They increased also in size of body ; their legs became less fine, and they lost their elegant carriage. Fresh eggs were then procured from wild birds ; but again the same result followed. In these cases of the duck and turkey we see that animals, like plants, do not depart from their primitive type until they have been subjected during several generations to domestication. On the other hand, Mr. Yarrell informed rue that the Australian dingos, bred in the Zoological Gardens, almost invariably produced in the first generation puppies marked with white and other colours; but these introduced dingos had probably been procured from the natives, who keep them in a semi-domesticated state. It is certainly a remarkable fact that changed conditions should at first produce, as far as we ca.n see, absolutely no effect; but that they should subsequently cause the character of the species to change. In the chapter on pangenesis I shall attempt to throw a little light on this fact. Returning now to the causes which are supposed to induce variability. Some authors 31 believe that close interbreeding gives this tendency, and leads to the production of monstrosities. In the seventeenth chapter some few facts were advanced, showing that monstrosities are, as it appears, occasionally thus caused; and there can be no doubt that close interbreeding induces lessened fertility and a weakened constitution ; hence it may lead to variability : but I have not sufficient evidence on this head. On the other hand, close interbreeding, if not carried to an injurious extreme, far from causing variability, tends to fix the character of each breed. It was formerly a common belief, still held by some persons, that the imagination of the mother affects the child in 3! Devay, '1\'Iariages Consanguins,' pp. D7, 125. In conversation I have found two or three natm·alists of the same opinion. |