258 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



ture into two groups, viz., artiodactyle or even-toed, and 

 perissodactyle or odd-toed. The artiodactyles may have 

 four toes, as in the hippopotamus and the hog, or two 

 toes, as in ruminants. The perissodactyles may have 

 three toes, as in the rhinoceros and tapir, or one toe only, 

 as in the horse and zebra. Both of these groups have 

 come from a five-toed ancestor by successive dropping 

 of toes. There is also a regular order in the dropping. 

 In the five-toed ancestor the two groups were not yet 

 differentiated. The first toe to drop was the thumb, 

 leaving the remaining four toes all functional, as in the 

 hippopotamus. Then, if evolution is on the artiodac- 

 tyle line, the two side toes gradually dwindle and shorten 

 up, as in the hog, and finally disappear, as in ruminants, 

 leaving only two greatly enlarged toes, which correspond 

 to the middle and ring fingers of man. But if, on the 

 other hand, the animal is on the perissodactyle line, 

 then, of the four toes, the little finger first dwindles, as 

 in the fore foot of the tapir, and then disappears, leaving 

 three. Now, of these three, the side toes dwindle and 

 shorten up and finally disappear, leaving only the great- 

 ly enlarged middle toe, as in the horse. 



6. Rudimentary Organs j Useless Organs. — All through 

 the animal kingdom, especially in the more specialized 

 forms of mammals, we find rudimentary and often 

 wholly useless organs. These are evidently remnants 

 of once useful organs, which have dwindled by disuse, 

 but have not yet entirely disappeared. Examples meet us 

 on every side, (a) The horse's toes are reduced to one, but 

 not at once. It was a gradual process, every step of which 

 may be traced. The first representative of the horse 

 in the line of its descent had five toes on the fore foot 

 then, in later times, four, then three. Three-toed horses 

 continued a long time, the two side toes meanwhile 

 dwindling and shortening up and finally passing away. 



