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POPULAR SCIENCE MONTHLY 



the two extra toes were developed by the variation of the rudimentary 

 pollex (Fig. 5, A-D). 



n. 



n. 



IV in. 



Fig. 6. A, calf's nianus 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 Eoman times the horse is known to have possessed extra toes. 

 Suetonius alludes to a horse given to Julius Caesar ' 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 com- 

 pletely 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 



