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habit, the result of domestication, and to the principle of 
correlated variation. 
“It is scarcely possible to read Nathusius’s excellent Vorstudien 
and doubt that, with the highly improved races of the pig, abundant 
food has produced a conspicuous effect on the general form of the 
body, on the breadth of the head and face, and even on the teeth. 
Nathusius rests much on the case of a purely bred Berkshire pig, 
which when two months old became diseased in its digestive organs, 
and was preserved for observation until nineteen months old ; at this 
age it had lost several characteristic features of the breed, and had 
acquired a long, narrow head, of large size relatively to its small 
body, and elongated legs.”-—(The Variation, vol. ii., p. 2yg.) 
“He (Nathusius) lays much stress on the fact that all wild and 
semi-domesticated pigs, in ploughing up the ground with their 
muzzles, have, whilst young, to exert the powerful muscles fixed in 
the hinder part of the head. In highly cultivated races this habit is 
no longer followed, and consequently the back of the skull becomes 
modified in shape, entailing other changes in other parts.”—(The 
Variation, vol. i., p. 72.) 
“ Nathusius has shown that, with the improved races of the pig, the 
shortened legs and snout, the form of the articular condyles of the 
occiput and the position of the jaws, with the upper canine teeth 
projecting in a most anomalous way in front of the lower canine, may 
be attributed to these parts not having been fully exercised. For the 
highly cultivated races do not travel much in search of food, nor root 
up the ground with their ringed muzzles. These modifications of 
structure, which are all strictly inherited, characterise several im 
proved breeds, so that they cannot have been derived from any single 
domestic or wild stock.”—(The Variatio7i. vol. ii., p. 2qq.) 
“ We shall, in a future chapter, also see that the skull and limbs 
are apparently in some manner correlated, so that any change in the 
one tends to affect the other.”—(The Variatioii. vol. i.,p. yj.) 
We have now to ask what part artificial selection has 
played in the production of this phenomenon. 
In the first place it is maintained by Mr. Darwin that the 
curly tail of the highly-bred pig cannot have been made 
the subject of selection ; so that in this organ we have an 
illustration of a modification not due to the principle of 
selection. It seems to me that we may apply the same