354 



LAWS OF VARIATION. 



Chap. XXVI. 



organ, such as the beak, increases or decreases in length, 

 adjoining or correlated parts, as the tongue and the orifice of 

 the nostrils, tend to vary in the same manner. "When the 

 whole body increases or decreases in size, various parts become 

 modified; thus with pigeons the ribs increase or decrease in 

 number and breadth. Homologous parts which are identical 

 during their early development and are exposed to similar condi- 

 tions, tend to vary in the same or in some connected manner, 

 as in the case of the right and left sides of the body, of the front 

 and hind limbs, and even of the head and limbs. So it is with 

 the organs of sight and hearing ; for instance white cats 

 with blue eyes are almost always deaf. There is a manifest 

 relation throughout the body between the skin and its various 

 appendages of hair, feathers, hoofs, horns, and teeth. In 

 Paraguay, horses with curly hair have hoofs like those of a 



mule ; the wool and the horns of sheep vary together ; hairless 



teeth; men with redundant hair 



dogs are deficient in their 





have abnormal teeth, either deficient or in excess. Birds with 

 long wing-feathers usually have long tail-feathers. When long 

 feathers grow from the outside of the legs and toes of pigeons, 

 the two outer toes are connected by membrane ; for the whole 



leg tends to assume the structure of the 



wmg 



There is a 



manifest relation between a crest of feathers on the head and 

 a marvellous amount of change in the skull of various fowls; 

 and in a lesser degree, between the greatly elongated, lopping 

 ears of rabbits and the structure of their skulls. With plants 

 the leaves, various parts of the flower, and the fruit, often vary 

 together in a correlated manner. 



? 



In some 



find correlation 



jecture what is the 



being able even 



of the connection, as with 



various correlated monstrosities and diseases. This is likewise the 

 case with the colour of the adult pigeon, in connection with 

 the presence of down on the 



young 



bird. Numerous curious 

 instances have been given of peculiarities of constitution, in 



correlation with colour, as shown by the immunity of individuals 

 of some one colour from certain diseases, from the attacks of 

 parasites, and from the action of certain vegetable poisons. 



Correlation is an important subject; for with species, and 

 in a lesser degree with domestic races, we continually find that 



