142 THE EVOLUTION OF MAN. 



including Man, must be traced from a single common mam- 

 malian parent-form. This long extinct primaeval root-form 

 and its immediate descendants which differ from each 

 other hardly more than do several species of one genus we 

 will call Primitive Mammals (Promammalia). As we have 

 already seen, this root-form developed from the ancient 

 parent-form of the Primitive Amnion Animals in a direction 

 wholly different from that followed by the Reptile group, 

 which afterwards gave rise to the more highly developed 

 class of Birds. The differences which distinguish Mammals 

 on the one side, from Reptiles and Birds on the other, are so 

 important and characteristic, that we may quite safely as- 

 sume a bifurcation of this kind in the vertebrate family tree. 

 Reptiles and Birds which we classed together as Monocon- 

 dylia, or Sauropsida coincide entirely, for instance, in the 

 characteristic structure of the skull and brain, which is 

 strikingly dissimilar from that of the same parts in Mam- 

 mals. In Reptiles and Birds, the skull is connected with the 

 first cervical vertebra (the atlas) by a single joint-process 

 (condyle) of the occipital bone ; in Mammals, on the con- 

 trary (as in Amphibians), the condyle is double. In the 

 former, the under jaw is composed of many parts, and is 

 connected with the skull by a peculiar bone of the jaw 

 (the square bone) so as to be movable ; in the latter, on the 

 contrary, the lower jaw consists of but two bone-pieces, 

 which are directly attached to the temporal bone. Again, 

 the skin of the Sauropsida (Reptiles and Birds) is covered 

 with scales or feathers, that of the Mammals with hair. 

 The red blood-cells of the former are nucleated, those of the 

 latter non-nucleated. The eggs of the former are very 

 large, are provided with a large nutritive yelk, and undergo 



