12 VARIATION Chap. I. 



cats with blue eyes are invariably deaf ; colour and con- 

 stitutional peculiarities go together, of which many 

 remarkable cases could be given amongst animals and 

 plants. From the facts collected by Heusinger, it ap- 

 pears that white sheep and pigs are differently affected 

 from coloured individuals by certain vegetable poisons. 

 Hairless dogs have imperfect teeth; long-haired and 

 coarse-haired animals are apt to have, as is asserted, long 

 or many horns ; pigeons with feathered feet have skin 

 between their outer toes ; pigeons with short beaks have 

 small feet, and those with long beaks large feet. Hence, 

 if man goes on selecting, and thus augmenting, any pe- 

 culiarity, he will almost certainly unconsciously modify 

 other parts of the structure, owing to the mysterious 

 laws of the correlation of growth. 



The result of the various, quite unknown, or dimly 

 seen laws of variation is infinitely complex and diversified. 

 It is well worth while carefully to study the several 

 treatises published on some of our old cultivated plants, 

 as on the hyacinth, potato, even the dahlia, &c. ; and it 

 is really surprising to note the endless points in struc- 

 ture and constitution in which the varieties and sub- 

 varieties differ slightly from each other. The whole 

 organisation seems to have become plastic, and tends to 

 depart in some small degree from that of the parental 

 type. 



Any variation which is not inherited is unimportant 

 for us. But the number and diversity of inheritable 

 deviations of structure, both those of slight and those of 

 considerable physiological importance, is endless., Dr. 

 Prosper Lucas's treatise, in two large volumes, is the 

 fullest and the best on this subject. No breeder doubts 

 how strong is the tendency to inheritance : like produces 

 like is his fundamental belief: doubts have been thrown 

 on this principle by theoretical writers alone. When a 



