ARTIFICIAL SELECTION. 1 35 



over, pre-eminently in the pigeon may be traced the 

 phenomenon which has been termed the " correlation 

 of growth," and consists in the fact that, with the in- 

 tentional modification of an organ by means of selection, 

 one or more other organs are drawn into sympathy and 

 unintentionally transformed into characteristics of a race. 

 Darwin's minute researches on the formation of races 

 in the pigeon are recounted in his second work on the 

 theory of Descent, "The Variation of Animals and Plants 

 under Domestication," in which the most detailed investi- 

 gations respecting other domestic animals are also to be 

 found. Whoever hr.s had occasion to inspect one of the 

 modern exhibitions of poultry, must have been astonished 

 at the diversity of the different races, and the purity and 

 uniformity within each race. Though not quite so posi- 

 tively as in the case of the pigeon, yet with approximate 

 certainty, the domestic fowl appears to be derived from 

 a single ancestral stock, the Indian Gallus Bankiva. The 

 cumulative power of selection by man is likewise testified 

 by the various races of pigs bred within the last century 

 by the English farmers from an intermixture of the 

 native and Indian races, differing in general appearance, 

 colouring, size of ears, length of legs, and also partially 

 in fertility. Our attention is, however, more closely 

 drawn to the two races of Southdown sheep and Short- 

 horn cattle, which, as well as the choicest breeds of pigs, 

 have been for some years past particularly esteemed on 

 the continent. These and many other races have been 

 bred with definite purposes, and for certain domestic and 

 commercial advantages, and one and all bear testimony 

 to the plasticity of species. 



Artificial selection operates by establishing peculiarities 



