CHAPTEE III 



ORIGIN AND CLASSIFICATION OF THE MAMMALIA 



Origin. — Although, as stated in the first chapter, the mammalian 

 class, as at present known either by existing or extinct forms, is 

 completely isolated from all other groups of the animal kingdom, 

 yet it is impossible to refrain from speculating as to its origin and 

 nearest affinities. In arranging the classes of vertebrates in a linear 

 series it is customary to place them in the following order — Pisces, 

 Amphibia, Reptilia, Aves, Mammalia, — an order which probably 

 indicates the relative degree of elevation to which the mos 

 highly developed members of each class has attained. Such 

 an arrangement appears to express the true relationship of the first 

 four classes to one another, but it is quite clear that the Mammalia 

 have no sort of affinity with the Aves. Writing in 1879, Professor 

 Huxley l came to the conclusion that, in looking among vertebrates 

 for the progenitors of the Mammalia, we must pass over all known 

 forms of birds and reptiles, and go straight clown to the Amphibia. 

 In addition to the characters derived from the conformation of the 

 pelvis upon which the argument was primarily based, the following 

 reasons were given for this conclusion : " The Amphibia are the 

 only air-breathing Vertebrata which, like mammals, have a dicon- 

 dylian skull. It is only in them that the articular element of the 

 mandibular arch remains cartilaginous, while the quadrate ossifica- 

 tion is small, and the squamosal extends down over it to the osseous 

 elements of the mandible, thus affording an easy transition to the 

 mammalian condition of those parts. The pectoral arch [girdle] of 

 the Monotremes is as much amphibian as it is sauropsidian ; the 

 carpus and the tarsus of all Sauropsida, except the Chelonia, are 

 modified away from the Urodele type, while those of the mammal 

 are directly reducible to it. Finally, the fact that in all Sauropsida 

 it is a right aortic arch which is the main conduit of arterial blood 

 leaving the heart, while in mammals it is a left aortic arch which 



1 Proceedings of the Royal Society of London, vol. xxviii. p. 395 (1879). 



