External and Internal Factors in Evolution 313 
the many domestic races of quadrupeds and birds belonging 
to different orders, with goldfish and silkworms, with plants 
of many kinds, raised in various quarters of the world. In 
the deserts of northern Africa the date-palm has yielded 
thirty-eight varieties ; in the fertile plains of India it is noto- | 
rious how many varieties of rice and of a host of other plants 
exist; in a single Polynesian island, twenty-four varieties of 
the breadfruit, the same number of the banana, and twenty- 
two varieties of the arum, are cultivated by the natives. The 
mulberry tree of India and Europe has yielded many varie- 
ties serving as food for the silkworm; and in China sixty- 
three varieties of the bamboo are used for various domestic 
purposes. These facts, and innumerable others which could 
be added, indicate that a change of almost any kind in the 
conditions of life suffices to cause variability — different 
changes acting on different organisms.” 
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 impor- 
tant 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 con- 
ditions is cumulative, in the sense that it may not appear 
until the species has been subjected to it for several genera- 
tions. 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 
