ARTIFICIAL SELECTION. 135 
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 has 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 
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