THEORETICAL BIOLOGY 



461 



Darwin made a special study of domesticated animals. It is a 

 fact of common observation that no two animals arc exactly alike, 

 and that man by selecting desired points of variation can produce 

 stocks in which these special points arc developed to a remark- 

 able degree. Thus by carefully selected variations the breeder 

 has produced the many varieties of domestic cattle, horses, and 

 fowls with which we are familiar. All these different breeds 

 are considered varieties of one and the same species, and each, it 

 kept isolated from other varieties, will in most cases breed true, 



Fin. 419. Columba //via, several varieties, showing the great variation under dome 

 Hon and artificial selection. (From a photograph provided by the American Museum of 



Natural History.) 



that is to say reproduce its kind with but slight variation ; but if 

 allowed to cross with other varieties, the distinctive character- 

 istics of each tend to disappear and there is usually a reversion 

 to the primitive ancestral type, a phenomenon known as atavism. 

 Darwin's experiments with the domestic pigeon afford an in- 

 structive example. Fanciers recognize a large number of 

 varieties in the pigeon, such as pouters, tumblers, carriers, and 

 fantails, and these are so different from one another that 



