53 6 ORIGIN OF MAMMALS xix. 2- 



cannot say exactly when they arose, and our technical definition of a 

 mammal must be made on the basis of hard parts. 



The whole series of mammal-like forms from Carboniferous anap- 

 sids onwards forms a natural unit, and it is only by an arbitrary 

 convention that we separate the reptilian subclass Synapsida from 

 the class Mammalia. The present-day mammals form a distinct group 

 of animals, which we identify superficially by their possession of hair. 

 For instance, the duck-billed platypus is immediately referred because 

 of its hair to the mammalia, and we class it apart from reptiles or 

 birds, even though its internal organization and the fact that it lays 

 eggs show it to have many similarities with reptiles, and its bill is like 

 that of a duck. The technical characteristic of the class Mammalia is 

 conventionally given by the presence of a single dentary bone in the 

 lower jaw, the articular and other bones forming, with the quadrate, 

 part of the mechanism of the middle ear. However, fossils showing 

 intermediate conditions are now known from the upper Trias, say 180 

 million years ago, and all stages can thus be traced in the jaws. The full 

 mammal-like condition was established by the middle Jurassic period, 

 about 150 million years ago. The reduction of the jaw bones may per- 

 haps have been associated with the habit of chewing the food, in order 

 to obtain the large amounts necessary to maintain a high temperature. 



It would not be very rash to suggest that by Jurassic times the 

 synapsids had developed the other mammalian characters, such as 

 active habits, large brain, warm blood, hair, and perhaps also a 

 diaphragm, four-chambered heart, and single left aortic arch. They 

 may even have been viviparous, for the surviving monotremes, which 

 lay eggs, probably diverged at a still earlier period, perhaps in the 

 Trias (p. 556). 



3. Mammals of the Mesozoic 



In spite of all the uncertainties of the fossil record it is now possible 

 to follow the history of the Mammalia back to their origin from coty- 

 losaurian reptiles of the Permian, more than 225 million years ago. 

 Sufficient information is available for us to be quite sure that some 

 population of early anapsid reptiles, such as * Seymouria of the Car- 

 boniferous and Permian times, besides giving rise to all the modern 

 reptiles, to the dinosaurs, and to the birds, also produced the mam- 

 mals. The evidence for this connexion rests on a most interesting 

 series of fossils, together with some 'living fossils' such as the mono- 

 tremes of Australia. The fossil history is not at all times equally clear. 

 The mammalian stock first became distinct in late Carboniferous 



