Darwin thinks that a change in climate alone is not one of the potent causes of variability, because the native country of a plant, where it has been longest cultivated, is where it has oftenest given rise to the greatest number of varieties. He thinks it also doubtful that a change in food is an important source of variability, since the domestic pigeon has varied more than any other species of fowl, yet the food has been always nearly the same. This is also true for cattle and sheep, whose food is probably much less varied in kind than in the wild species. Another point of interest is raised by Darwin. He thinks, as do others also, that the influence of a change in the conditions is cumulative, in the sense that it may not appear until the species has been subjected to it for several generations. Darwin states that universal experience shows that when new plants are first introduced into gardens they do not vary, but after several generations they will begin to vary to a greater or less extent. In a few cases, as in that of the dahlia, the zinnia, the Swan River daisy, and the Scotch rose, it is known that the new variations only appeared after a time. The following statement by Salter is then quoted, “Every one knows that the chief difficulty is in breaking through the original form and color of the species, and every one will be on the lookout for any natural sport, either from seed or branch; that being once obtained, however trifling the change may be, the result depends on himself.” Jonghe is also quoted to the effect that “there is another principle, namely, that the more a type has entered into a state of variation, the greater is the tendency to continue doing so, and the more it has varied from the original type, the more is it disposed to vary still further.” Darwin also quotes with approval the opinion of the most celebrated horticulturist of France, Vilmorin, who maintained 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.” Darwin also cites a few cases where animals have changed quite quickly when brought under domestication. Turkeys raised from the eggs of wild species lose their metallic tints, and become spotted with white in the third generation. Wild ducks lose their true plumage after a few generations. “The white collar around the neck of the mallard becomes much broader and more irregular, and white feathers appear in the duckling’s wings. They increase also in size of body.” In these cases it appears that several generations were necessary in order to bring about a marked change in the original type, but the Australian dingoes, bred in the Zoological Gardens, produced puppies which were in the first generation marked with white and other colors.