142 GRADUAL MODIFICATION OF ANIMAL FORMS xi 



The order includes the most familiar of our domestic 

 animals, and with the general appearance of the rest we are 

 most of us well acquainted, thanks to the Zoological Gardens. 

 They are the various forms of horses, asses, and zebras, the 

 rhinoceroses and the tapirs, the pigs, hippopotamus, camels, 

 deer, antelopes, sheep, oxen, and goats. 



They are essentially herbivorous (though some few may be 

 more or less omnivorous), and their teeth are modified 

 accordingly. Their limbs are adapted for carrying the body 

 in ordinary terrestrial progression, and are of very little use for 

 any other purpose, such as climbing, seizing prey, or carrying 

 food to the mouth. They never have clavicles or collar-bones, 

 and their toes never exceed four in number (the digit which 

 corresponds to the first of the complete pentadactyle foot being 

 always wanting), and have the ends encased in hoofs instead 

 of nails or claws. The species at present existing are very 

 numerous, and widely diffused over the earth's surface, being 

 wanting only in the Australian province. These Ungulate 

 animals are divided into two natural groups, each having very 

 many characters in common, the establishment of which, though 

 contrary to the views of the great naturalists of the beginning 

 of this century, has been a great gain to zoological science, 

 especially as this division pervades all the known extinct as 

 well as recent forms ; and although some forms of either group 

 may present some partial approximation to the other, no 

 directly intermediate species are known. It is important, 

 therefore, to apprehend thoroughly the distinction between 

 these groups, which have received from Professor Owen the 

 names of Perissodactyle, or odd-toed, and Artiodactyle, or even- 

 toed, from one of their most striking external characteristics. 

 The first have the toes of both feet arranged symmetrically 

 to a line drawn through the middle of what would be the 

 third toe of the typical pentadactyle foot, which toe is always 

 the largest, and in some cases the only one fully developed. 

 In the second, the toes are arranged symmetrically to a line 

 drawn between the third and fourth toes, so that these two 

 toes are equally developed, and may be alone present, or may 

 be supplemented by an outer pair (the second and fifth), often 

 in a more or less rudimentary condition. Besides these dis- 



