MAMMALIA 333 



come from either Amphibia or birds. The latter possibility is out 

 of the question, for the birds are more recent than the mammals. 

 There is, however, some ground for the idea that the mammals may 

 have been derived directly from the Amphibia. 



The Theory of Amphibian Ancestry of the Mammals. — This 

 theory was advanced by Huxley and gained considerable vogue until 

 recent discoveries eliminated it from the field. Huxley's argument 

 in brief was as follows: The presence of two occipital condyles sep- 

 arates the mammals from the reptiles, and unites them with the 

 Amphibia. The mammals retain the left aortic arch and lose the 

 right, while birds retain the right arch and lose the left. Reptiles 

 show a tendency to reduce the left arch, which does not look to- 

 ward a mammalian condition, and therefore discredits the reptile 

 ancestry idea. 



This theory is based on the supposition that the condyles and 

 aortic arches of modern reptiles are primitive and were the same in 

 the early reptiles as they are to-day. This is a fallacy, however, 

 for some of the early reptiles, notably the cynodonts, had two con- 

 dyles Uke the Amphibia. It is also quite possible that the early 

 reptilio-mammal stock had a reversed symmetry of the aortic 

 arches. Although still advocated by some modern writers, the 

 theory of amphibian ancestry of the mammals confidently may 

 be set aside. 



PalaBontological Evidence of the Origin of the Mammals. — Within 

 comparatively recent years the fossil evidences of mammalian de- 

 scent have been vastly strengthened by the discovery in Triassic 

 rocks of South Africa of a large collection of remains of a group of 

 extinct reptiles known as Cynodontia (dog-toothed), that have al- 

 ready been dealt with in the chapter on reptiles. There were niany 

 types of cynodonts, some of which exhibit mammalian tendencies of 

 one sort, others of another. Certain authorities claim that all of the 

 distinctions between reptiles and mammals, based on bony structures, 

 are transgressed by one or more groups of cynodonts, some groups 

 transgressing with regard to a majority of distinctions, other groups 

 with regard to one or a very few. These creatures were obviously 

 reptiles of a rather generalized type in most respects, but they were 

 evidently making some of the same experiments that the ancestral 

 mammals must have made in order to arrive at the present mamma- 

 lian status. Whether or not these cynodonts were the actual an- 



