574 PLACENTAL MAMMALS xxi. 4- 



and South Africa, and the Oriental regions are capes projecting into 

 the sea from the central Palaearctic (Eurasian) land-mass. There is 

 evidence that many forms of animal life evolved in this central area 

 and migrated away towards the extremities. Several of the types 

 that evolved earlier remain as vestiges at these 'ends of the world', 

 long after newer types have replaced them nearer to the centre. The 

 lung-fishes are a conspicuous example of this and at another extreme 

 it is perhaps no accident that Australian Aborigines and South African 

 Bushmen are among the more primitive of men. This is the most prob- 

 able explanation of the similarity in the fauna between these southern 

 continents, but many suggestions of Antarctic land bridges and con- 

 tinental drift have been put forward, in particular one connecting 

 South Africa and South America in the early Tertiary. For example, 

 we have to explain the appearance of porcupine-like animals (p. 660) 

 in Africa and South America at the same time (Oligocene). 



5. The earliest eutherians 



Fossil placentals found in the Cretaceous period have all been in- 

 sectivorans (p. 583). Those of the Palaeocene include also some that 

 can be referred to the primates and to the carnivora, but they are very 

 unlike modern members of those groups and could almost equally 

 well be classed as Insectivora. Similarly, the ungulates and various 

 other types that appeared in the Palaeocene could have had an in- 

 sectivoran ancestry. Evidently, therefore, during the late Cretaceous 

 and Palaeocene, the original placental stock was branching out into 

 various habitats, and the branching was rapid. Simpson's careful 

 classification recognizes twenty-six orders of placentals (p. 577), and 

 nearly all of these had become distinct by the Eocene. Ten of them 

 have since become extinct. Evidently the great period of mammalian 

 expansion was in the earlier part of the Cenozoic and the group may 

 be considered to have passed its peak for the present. Only the bats, 

 rodents, lagomorphs, and perhaps the primates and carnivores can 

 be considered really successful land animals at the present time; to 

 these we may add the whales in the sea. The Artiodactyla are also 

 abundant, but most of the remaining placental orders are today 

 poorly represented in numbers. 



In order to gain a comprehensive general understanding of the great 

 placental history during its 80 million or so years duration, we will 

 first give a technical definition of a placental, then the characteristics 

 of the earlier types, and finally try to list some of the tendencies to 

 change that are widely found in different groups. For simplification 



