the two extra toes were developed by the variation of the rudimentary pollex (Fig. 5, A-D).
Fig. 6. A, calf's manus with digit II. fully developed (from X-ray photograph); B, manus or sheep with two extra digits (II. and V.) present (after Chauveau); C, manus of horse with two extra digits (I. and II.) (after Marsh).
The writer has also observed two cases in which the second digit of the ox was developed into a functional toe (Fig. 6, A), and in the foot of the sheep four complete digits sometimes occur (Fig. 6, B). As far back as Roman times the horse is known to have possessed extra toes. Suetonius alludes to a horse given to Julius Cæsar 'which had feet that were almost human, the hoofs being cleft like toes.' Two cases were described by Winter in 1703, and Marsh has since observed the development of an extra digit from one of the splint bones (Fig. 6, C); four or five digits may sometimes occur, but all of these are not completely developed.
It is thus clear that the vestiges regarded as digital rudiments are really such, and that mammals possessing these vestiges must at one time have had a greater number of functional toes, some of which later became useless. It is a well-known theory that this reduction in the number of digits was in adaptation to some special function like that of locomotion. It has been carried to the extreme in the foot of the hoofed mammals; and of living forms, the swine and ruminants afford a beautiful series of digital reductions (Fig. 7, A-H). Even among living carnivora, forms like the cat and dog have the pollex reduced and the hallux absent, and, as we have seen, the forerunners of the swine had a reduced pollex on the manus (Fig. 7, A), and only four digits on the pes. The first digit is vestigial among the hippopotami; the second and fifth are slightly smaller than the third and fourth (Fig. 7, B). The difference in the size of the two pairs of digits is more marked in another fossil pig, but the small outer digits still articulate firmly at the wrists and ankle joints (Fig. 7, C). The third and fourth toes of the swine are relatively much larger and have taken
