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 



