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 Speczes (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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