THE PHYLOGENESIS OF VERTEBRATES 



37 



which resembled the present-day Insectivora, passed over into 

 that Order. 



This insectivorous stem, in addition to perfecting its own 

 type along the narrow lines first laid down, developed several 

 lines of differentiation, and it was from these that all the 

 higher placental mammals have arisen. A very primitive 

 stem is that of the Rodentia, of which the extinct group of 

 Tillodontia may have been the first; succeeded by the Du- 

 plicidentata or gnawing animals, like the rabbits, in which, 

 back of the two sharp upper incisors, there is a second re- 

 duced pair, and later by the Simplicidentata, like squirrels, 

 rats, mice, and beavers, in which the upper incisors consist 

 of a single pair. 



The branch represented here as immediately above the last, 

 suggesting a little less primitive character, is that leading to 

 the group usually called the Edentata, and consisting of the 

 sloths, armadilloes, ant-eaters, besides several extinct forms, 

 such as the Megatherium, Megalonyx, and Glyptodon, the 

 first two like the sloths, the last like an armadillo. In the 

 more specialized of these there is a peculiar joint between 

 two of the vertebrae of the back, and they are called the Xenar- 

 thra in contrast to those in which this joint is normal, the No- 

 marthra. This group has always been exclusively American, 

 the living forms mainly South American. 



From this same generalized group, the Insectivora, there 

 have developed two distinct lines of flying or soaring forms, 

 the Chiroptera or bats, and the Galeopithecus, a single species 

 found in Madagascar, but not nearly related to any of the 

 other stems. 



By far the most prolific of the stems proceeding from the 

 Insectivora is that which started with the extinct group of 

 Creodonta. These animals were at first small, generalized 

 mammais, scarcely distinguishable from the parent insecti- 

 vores, but they gradually took on special characters which 

 suggest the modern Carnivora, which are considered their 

 direct descendants. Before specializing along this line, how- 

 ever, some of them began to differentiate in several other di- 



