ADAPTATION. I75 



SO. But as the shepherd unerringly knows the phy- 

 siognomy of his sheep where an excursionist from the 

 town sees only a general sheep's face, so to an attentive 

 naturalist, in most of the lower organisms, the specific 

 type resolves itself into as many varieties as individuals, 

 irrespectively of the cases in which no specific type can 

 be established. 



As modification under given conditions, adaptation is 

 thus as little an unknown quantity as heredity, but is 

 merely a function of the mechanical character of muta- 

 bility, or, in the widest sense of the word, of nutrition. 

 Adaptation takes place when the organism or its parts 

 are pliable and plastic to external influences, when they 

 conquer and make use of them. Climate, light, humidity, 

 nutriment, are hindrances or advantages that directly or 

 indirectly affect the organism, and are all actively con- 

 cerned in it. Surrounded by organisms, we see them 

 without exception adapting themselves to circumstances; 

 and if our only object is to be convinced of the formative 

 influence of the mode of life, this is most readily done 

 in the case of domestic animals. In his studies on the 

 pig, H. von Nathusius, perhaps the most scientific of the 

 celebrated breeders, shows how in the simplest cases, 

 where the looseness of cultivated soil has facilitated the 

 labour of grubbing, the skull of the domestic pig is ar- 

 rested, by the softer structure of the cranium, at the 

 immature form of the wild boar, and how those extreme 

 shapes of the head in cultivated breeds, characterized by 

 the bending and shortening of the face, and the impossi- 

 bility of closing the jaw in front, are entirely the result 

 of their altered mode of life. It is known that men, 

 animals, and plants, removed far from their previous 



