ORIGIN OF MAMMALS 237 



Following Cuvier, Owen, and Huxley in Europe, a period 

 of active research in this country began with Leidy in the 

 middle of the nineteenth century and was continued in the 

 arid regions of the West by Cope, Marsh, and their succes- 

 sors with such energy that America has become the chief cen- 

 tre of vertebrate palaeontology. When we connect this research 

 with the older and the more recent explorations by men of all 

 countries in Europe, Asia, Africa, Australia, and South Amer- 

 ica, we are enabled to reconstruct the great tree of mammalian 

 descent (Fig. 114) with far greater fulness and accuracy than 

 that of the reptiles, amphibians, or fishes (Pisces). 



The connection of the ancestral mammals with a reptilian 

 type of Permian time is theoretically established through the 

 survival of a single branch of primitive egg-laying mammals 

 (Monotremata, Fig. 113) in Australia and New Guinea; while 

 the whole intermediate division, consisting of the pouched 

 mammals (Marsupialia) of Australia, which bring forth their 

 young in a very immature condition, represents on the great 

 continent of AustraHa an adaptive radiation which also sprang 

 from a small, primitive, tree-living j Whales. 

 type of mammal, typified by the ex- 2. Seals (marine carnivores). 



... r AT ^-u J c <-i, S- Carnivores (terrestrial). 



istmg opossums of North and bouth -^ -, . 



° ^ 4. Insectivores. 



America (Fig. 113). The third great 5. Bats, 

 group (Place ntalia) includes the 6. Primates: 



1 • I,- 1 4^1, u Lemurs, 



mammals m which the unborn Monkevs 



young are retained a longer period Apes, 



within the mother and are nourished Man. 



, , . , . . . . 7. Hoofed mammals, 



through the circulation 01 nutrition g j^^^atees 



in the placenta. 9. Rodents. 



The adaptive radiation of the ten ^°- Edentates. 

 great branches of the placental stock from the primitive insec- 

 tivorous arboreal ancestors produced a mammalian fauna which 



