of the head of a wild boar and of a sow from a photograph of the Yorkshire Large Breed, may aid in showing how greatly the head in a highly cultivated race has been modified and shortened.
Fig. 3.—Head of Wild Boar, and of "Golden Days," a pig of the Yorkshire Large Breed; the latter from a photograph. (Copied from Sidney's edit. of 'The Pig,' by Youatt.)
Nathusius has well discussed the causes of the remarkable changes in the skull and shape of the body which the highly cultivated races have undergone. These modifications occur chiefly in the pure and crossed races of the S. Indica type; but their commencement may be clearly detected in the slightly improved breeds of the S. scrofa type.[[160]] Nathusius states positively (s. 99, 103), as the result of common experience and of his experiments, that rich and abundant food, given during youth, tends by some direct action to make the head broader and shorter; and that poor food works a contrary result. He 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 to 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. There can hardly be a doubt that so great a change in habits would
affect the skull; but it seems rather doubtful how far this will account for the greatly reduced length of the skull and for its concave front. It is well known (Nathusius himself advancing many cases, s. 104) that there is a strong tendency in many domestic animals—in bull- and pug-dogs, in the niata cattle, in sheep, in Polish fowls, short-faced tumbler pigeons, and in one variety of the carp—for the bones of the face to become greatly shortened. In the case of the dog, as H. Müller has shown, this seems caused by an abnormal state of the primordial cartilage. We may, however, readily admit that abundant and rich food supplied during many generations would give an inherited tendency to increased size of body, and that, from disuse, the limbs would become finer and shorter.[[161]] 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.
Nathusius has remarked, and the observation is an interesting one, that the peculiar form of the skull and body in the most highly cultivated races is not characteristic of any one race, but is common to all when improved up to the same standard. Thus the large-bodied, long-eared, English breeds with a convex back, and the small-bodied, short-eared, Chinese breeds with a concave back, when bred to the same state of perfection, nearly resemble each other in the form of the head and body. This result, it appears, is partly due to similar causes of change acting on the several races, and partly to man breeding the pig for one sole purpose, namely, for the greatest amount of flesh and fat; so that selection has always tended towards one and the same end. With most domestic animals the result of selection has been divergence of character, here it has been convergence.[[162]]
The nature of the food supplied during many generations has apparently affected the length of the intestines; for, according to Cuvier,[[163]] their length to that of the body in the wild boar is as 9 to 1,—in the common domestic boar as 13.5 to 1,—and in the Siam breed as 16 to 1. In this latter breed the greater
length may be due either to descent from a distinct species or to more ancient domestication. The number of mammæ vary, as does the period of gestation. The latest authority says[[164]] that "the period averages from 17 to 20 weeks," but I think there must be some error in this statement: in M. Tessier's observations on 25 sows it varied from 109 to 123 days. The Rev. W. D. Fox has given me ten carefully recorded cases with well-bred pigs, in which the period varied from 101 to 116 days. According to Nathusius the period is shortest in the races which come early to maturity; but in these latter the course of development does not appear to be actually shortened, for the young animal is born, judging from the state of the skull, less fully developed, or in a more embryonic condition,[[165]] than in the case of common swine, which arrive at maturity at a later age. In the highly cultivated and early matured races, the teeth, also, are developed earlier.
The difference in the number of the vertebræ and ribs in different kinds of pigs, as observed by Mr. Eyton,[[166]] and as given in the following table, has often been quoted. The African sow probably belongs to the S. scrofa type; and Mr. Eyton informs me that, since the publication of his paper, cross-bred animals from the African and English races were found by Lord Hill to be perfectly fertile.
|
English Long-legged Male. |
African Female. |
Chinese Male. |
Wild Boar, from Cuvier. |
French Domestic Boar, from Cuvier. | |
| Dorsal vertebræ | 15 | 13 | 15 | 14 | 14 |
| Lumbar | 6 | 6 | 4 | 5 | 5 |
| Dorsal and lumbar together | 21 | 19 | 19 | 19 | 19 |
| Sacral | 5 | 5 | 4 | 4 | 4 |
| Total number of vertebræ | 26 | 24 | 23 | 23 | 23 |