178 EVOLUTION AND ANIMAL LIFE 



ments or splints. This animal differs from the present horse 

 somewhat in skull, shape of hoof, length of teeth, and other 

 minor details. Lastly came the present horse, Equus, with the 

 splint bones or concealed rudiments of the side toes very small 

 and the hoof of the middle toe rounder. In spite of the great 

 difference between the one-toed foot of the living horse and 

 the dog's five-toed foot there was once a kind of horse which 

 had a five-toed foot, and there is after all a close relationship 

 between the foot of the horse and the foot of the dog. 



FIG. 112. Homology of digits of four odd-toed mammals, showing gradual reduction 

 in number and consolidation of bones above. (After Romanes.) 



In man there is developed at the proximal end of the caecum 

 or blind sac of the large intestine a small structure as shown in 

 Fig. 113. This appendage has no function, and it is subject to 

 inflammation or suppuration, known as appendicitis. In the 

 embryo the appendix vermiformis is notably larger than in 

 the adult man; and in the lower animals, as in the dog or the 

 kangaroo (see Fig. 113), it may be recognizable as a prolon- 

 gation of the caecum, scarcely less in diameter than the intestine 

 itself. The appendix vermiformis is therefore a vestige of a 

 long caecum which had its part in the process of digestion. 



In the embryo of all chordate animals, without exception, 

 respiratory or gill slits are developed, homologous with those 

 seen in the embryo of the fish. The presence of these slits or 

 their vestiges is one of the most important secondary distinctive 

 characters of the great group of Chordata, which includes the 

 vertebrates. The human embryo is, in this regard, at certain 



