134 



COMPARATIVE ANATOMY 



phylogenetic development of the fore-foot of the Horse, showing how it 

 has been gradually derived from a tetra- or pentadaotyle form; and it has 



recently been ascertained that all 

 these stages are passed through in the 

 course of ontogeny. In this case the 

 third digit becomes greatly enlarged 

 relatively (perissodactyle form), and 

 eventually is the only one remaining, 

 while in cloven-footed Ungulates the 

 third and fourth digits are both 

 functional and equally strongly de- 

 veloped (artiodactyle form) and may 

 be united together to form a "can- 

 non-bone," the others becoming 

 gradually reduced. A similar re- 

 duction takes place in the hind- 

 foot, and is here as a rule more 

 rapid. 



Ungulates diverged into Artio- 

 dactyles and Perissodactyles as far 

 back as the Eocene period, but a 

 large series of Tertiary forms shows 

 that they must all have been derived 

 from a common pentadactyle ances- 

 tral form. 



Some of the many other adaptive 

 modifications of the limbs in Mam- 

 mals must also be briefly mentioned. 

 In Bats, the phalanges are greatly 

 elongated to support the wing- 

 membrane ; the hallux as weU as 

 the pollex may be opposable amongst 

 the Primates ; the fore-limbs are 

 modified for digging in certain 

 Mammals {e.g. Mole) ; and in the 

 Cetacea (see p. 133) and Sirenia the 

 digits are not free, and serve as 

 supports for the fin-like paddles. 

 Nails are present on the digits of 

 Sirenia, but have disappeared in the Cetacea, though they can still he 

 recognised in the embryo of toothed Whales. Hind-limbs are absent 



Fig. 114. — Skeleton of the Left 

 Fore-Limb or A, Pig; B, Htomos- 

 CHus ; 0, Tbagulus ; Z>, Roebuck ; 

 E, Sheep ; F, Camel. (From Bell, 

 after Garrod. ) 



1 



VIVji II 



yrym a 



Fig. 115. — Foke-Foot of Ancestral Forms of the Horse. 1. Orohippus 

 (Eocene). 2. Mbsohippus (Upper Eocene). 3. Mtohippus (Miocene). 

 4. Pbotohippds (Upper Pliocene). 5. Phohippus (Uppermost Pliocene). 

 6. Equhs. 



in the two last mentioned Orders, but indications of them can be seen even 

 externally in very young embryos of the Porpoise, and rudiments of the thigh 

 and even shank bones occur in the adult in certain Whales (comp. p. 121). 



