The Making of Species 



Let us now consider a factor in evolution 

 which is nearly as important as natural selection 

 itself we allude to the phenomenon of correlation. 



CORRELATION 



We may define correlation as the inter- 

 dependence of two or more characters. This 

 phenomenon is far more common than the 

 majority of naturalists seem to think. It very 

 frequently happens that one particular character 

 never appears in an organism without being 

 accompanied by some other character which we 

 should not expect to be in any way related to it. 



Darwin called attention to this phenomenon. 

 "In monstrosities," he writes, on page 13 of the 

 Origin of Species (new edition), "the correlations 

 between quite different parts are very curious, 

 and many interesting instances are given in 

 Isidore Geoffroy St Hilaire's great work on this 

 subject. Breeders believe that long limbs are 

 almost always accompanied by an elongated head. 

 Some instances of correlation are quite whimsical : 

 thus cats which are entirely white and have blue 

 eyes are generally deaf; but it has been lately 

 stated by Mr Tait that this is confined to the 

 males. 



" Colour and constitutional peculiarities go 

 together, of which many remarkable cases could 

 be given among animals and plants. From the 



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