ADAPTATION, : 175 
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 
