THE PHYLOGENESIS OF VERTEBRATES 39 



James Bible as the " coney." It frequents Syria and the ad- 

 joining countries, and a related species is found in South 

 Africa ; the sole survivors of the early ungulates. 



The modern ungulate forms, aside from Hyrax, may be 

 represented by two stems, the one leading to the Proboscidea 

 and Sirenia, the other branching immediately into the Peris- 

 sodactyla, with an odd number of functional digits, and the 

 Artiodactyla, with an even number. The Proboscidea include 

 the two species of living elephants, besides several extinct 

 ones, like the mammuth, the mastodon, and the dinotherium, 

 and the Sirenia consist of two living genera of unwieldly 

 aquatic herbivores, the manatee or sea-cow, and the dugong, 

 which subsist on sea-weeds and consequently do not wander 

 far from the coasts. The Perissodactyla include the three 

 lines represented by the tapir, the rhinoceros, and the horse; 

 and the Artiodactyla embrace the non-ruminant pigs and 

 hippopotami, and the almost numberless species of ruminants, 

 such as cattle, sheep, antelopes, and deer. Of these perhaps the 

 most distinct are the giraffes, and the Tylopoda, or camels. 



In reviewing the two phylogenetic trees as given in Figs. 

 7 and 8, it will be seen that it is precisely those forms that 

 are the most needed to show the interrelationships of groups 

 that have suffered the most from the extinction of their species, 

 which is but another way of expressing the fact that general- 

 ized and transition forms are not as well fitted for the struggle 

 for existence as are their more specialized and better adapted 

 descendants, and are hence often exterminated by the very 

 races which have developed from them. This extermination 

 tends to isolate the terminal groups and thus to disguise the 

 plan of development, as may be seen by reference to Fig. 7, 

 in which the distinction is shown between living and extinct 

 groups. The effect of extinction will here be shown if the 

 reader imagines the extinct groups completely blotted out, 

 which will leave the modern orders entirely cut off from one 

 another. 



The same principle may be seen also in the second diagram, 

 the phylogenetic tree of mammals (Fig 8). Here the groups 



