ANCESTRY OF THE MAMMALIA 45 



The mammalian skull shows two laterally placed occipital con- 

 dyles whereas the Reptile possesses a single median condyle 

 ventral to the foramen magnum. 



During recent years Dr. Broom and Mr. Watson have given 

 much attention to the subject of premammalian Reptiles and 

 the former has summarized in a tentative but attractive man- 

 ner various features of the probable evolution of Mammals. 

 We owe largely to Broom the elucidation of a group of fossil 

 carnivorous Reptiles found in South Africa and belonging to 

 Permian and Triassic times. In the earlier period the Thero- 

 cephalia alone were present ; in the Triassic the Cynodonts also 

 appeared. It is undoubtedly from one or other of these related 

 groups that the mammalian ancestor arose but it must not be 

 supposed on that account that the first Mammals appeared in 

 Africa. From the history of the Insectivora and other mam- 

 malian orders we learn that the early Mammals very probably 

 evolved in the Northern hemisphere and that the less adaptable 

 members of the class, as previously suggested, migrated south- 

 ward when the climate in the north changed. Upon this hypoth- 

 esis we should find in South Africa only the more specialized 

 primitive members of the group. We do not look to any one of 

 these South African forms as the actual mammalian ancestor 

 but we know that in many features they must closely resemble 

 it. Existing Fishes and Reptiles, none of which are at all 

 close to the hypothetical line of mammalian descent, have al- 

 ready taught us certain lessons in the phylo genetic development 

 of tooth forms. Thus it is with the South African fossils: none 

 of them represents the actual mammalian ancestor but from 

 them we can reconstruct stages in mammalian evolution with a 

 fair degree of probable accuracy. 



In the Therocephalians the first maxillary tooth became 

 caniniform and with this change the temporal muscles in- 

 creased considerably in size and strength. In its turn the 

 growth of the temporal muscle brought about an enlargement 

 of the temporal fossa and induced the appearance of the coro- 

 noid process in the mandible. A zygomatic arch of mammalian 



