CHAPTER 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 + 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 down 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 Soctety of London, vol. xxviii. p. 395 (1879). 
