CAUSES OF VARIATION 127 



surrounding conditions are apparently quite 

 uniform ; as in bud variation in plants. 



The most variable of domesticated birds in Europe 

 are the pigeon, the fowl, and the duck. Of these 

 the pigeon and the duck are natives of Europe, while 

 the fowl is a native of India. On the other hand, 

 the goose , the guinea-fowl, the turkey and the 

 peacock have hardly varied at all in Europe ; and 

 yet the guinea-fowl comes from Africa, the turkey 

 from America, and the peacock from India. Evi- 

 dently change in external conditions is not a suffi- 

 cient explanation of variation ; and, as Darwin 

 maintained, individual constitution is of importance. 

 If varying the external conditions made plants and 

 animals vary, it would make the work of horticul- 

 turists and breeders much more easy than it is. 

 But they merely keep the animals or plants well 

 nourished and await results. Mr. H. Spencer has 

 however come to quite an opposite opinion from 

 that of Mr. Darwin, and finds in the environment 

 the principal cause of variation. 



That variations are not altogether due to physio- 

 genesis we see in the difference between brothers 

 and sisters of the same litter, or beween the seeds 

 from the same capsule. However, external condi- 

 tions must have some effect on organisms. The 

 birds and bees introduced into New Zealand from 

 the northern hemisphere readily changed their times 

 of breeding in accordance with the new seasons. 

 Most plants have done the same; but some, which 

 are reproduced by bulbs or offsets, take time to 

 accommodate themselves to the change. Gentiana 



