CORRELATED VARIATIONS. 85 



Besides these instances in which the degree of corre- 

 lation can be expressed in numerical equivalents, there 

 remain other cases in which such expression is difficult 

 or impossible. In these it must accordingly be de- 

 fined in general terms. Darwin has collected a large 

 number of such cases in his " Animals and Plants," * 

 but it is not necessary to reproduce more than a few of 

 the most striking of them here. For instance, Teget- 

 mier has stated that young pigeons of all breeds which 

 when mature have white, yellow, silver, blue, or dun- 

 coloured plumage, are born almost naked; whereas 

 pigeons of other colours are clothed with plenty of 

 down. Darwin himself has noticed that in feather- 

 footed pigeons, not only does the exterior surface sup- 

 port a row of long feathers, like wing feathers, but the 

 very same digits which in the wing are completely 

 united by skin become partially united by skin in the 

 feet. Again, Polish fowls have a large tuft of feathers 

 on their heads, and their skulls are perforated by 

 numerous holes. That this deficiency of bone is in some 

 way connected with the tuft of feathers is clear from 

 the fact of tufted ducks and geese likewise having per- 

 forated skulls. Constitutional peculiarities are some- 

 times correlated with colour in a most curious and inter- 

 esting manner. For instance, Beddoe has shown that a 

 relation exists between liability to consumption and the 

 colour of the hair, eyes, and skin. As regards ani- 

 mals, white terriers suffer most from distemper, white 

 chickens from a parasitic worm in their tracheae, white 

 pigs from scorching in the sun, and white cattle from 

 flies. Again, all the hogs in Virginia, excepting those 



*Cliap. xxv. 



