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 mammae vary, as does the
period of gestation. The latest authority says (3/21. S. Sidney 'The Pig'
page 61.) 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 the course of their 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 (3/22. 'Schweineschadel' s. 2, 20.) than in the case of
common swine. In the highly cultivated and early matured races the teeth,
also, are developed earlier.

The difference in the number of the vertebrae and ribs in different kinds
of pigs, as observed by Mr. Eyton (3/23. 'Proc. Zoolog. Soc.' 1837 page 23.
I have not given the caudal vertebrae, as Mr. Eyton says some might
possibly have been lost. I have added together the dorsal and lumbar
vertebrae, owing to Prof. Owen's remarks ('Journal Linn. Soc.' volume 2
page 28) on the difference between dorsal and lumbar vertebrae depending
only on the development of the ribs. Nevertheless the difference in the
number of the ribs in pigs deserves notice. M. Sanson gives the number of
lumbar vertebrae in various pigs; 'Comptes Rendus' 93 page 843.), 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 this paper, cross-bred animals from the African
and English races were found by Lord Hill to be perfectly fertile.