1 82 THE DARWINIAN THEORY 



and of the coccyx is also similar to that of reptiles ; 

 and the development of feathers is comparable with 

 that of the scales of a reptile and very unlike the 

 hairs of a mammal. 



Mammalia. — Mammals are characterised by being 

 warm-blooded animals with the following distinctive 

 points : — A heart with four cavities ; circular blood 

 corpuscles ; a diaphragm ; a left aortic arch ; a more 

 perfect, brain, especially as regards commissures, than 

 the other groups ; no quadrate bone ; three special 

 ear bones ; two occipital condyles ; mammary glands 

 and hairs. The anterior limbs are always present, 

 though the posterior may be absent. There are 

 almost invariably seven cervical vertebrae. Great 

 difference in size is met with among mammals, from 

 the harvest mouse to the whale. They form by far 

 the most important group of all animals from an 

 economic standpoint, some of them serving as food, 

 some furnishing clothing, and others being used for 

 transport. 



The geological history of mammals commences 

 with small forms found in the Trias, but compara- 

 tively scanty remains are found till the tertiary 

 period. From tertiary times onwards, they are 

 found in three great areas, very distinct from one 

 another: (i) Australia; (2) South America; (3) 

 Europe, Asia, Africa, and North America. 



The present distribution of Mammals is divided 

 into three great groups : 



(1) Monotremata: a very small group of lowly- 

 organised animals, confined to the Australian region. 

 (2) Marsupials: formerly wide-spread, but now 



