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THE CHANGING GENERATIONS 



The placental mammals. The young of the placental mammals are 

 retained within the body of the mother for a long time, while they pass 

 through their embryonic and fetal stages of development. During this 

 period they lead a parasitic existence, nourished within the uterus by 

 means of an organ called the placenta, formed from the old reptilian 

 allantois. For a time after birth they are cared for by the mother and fed 

 upon milk secreted by her mammary glands. Not only is their chance of 

 survival better than that of the young of egg-laying and marsupial 

 mammals, but the prolonged period of embryonic development makes 

 possible and is required for the attainment of the higher levels of com- 



Jl 



Fig. 28.29. The hind feet of a series of marsupials, showing the extraordinary history of the 

 feet of the tree kangaroo, Dendrolagus. The forms shown do not represent the actual 

 ancestral line but illustrate its successive stages. A, Caenolestes, a primitive walker. B, 

 opossum, a primary climber. C, phalanger, a specialized climber. D, bandicoot, a secondary 

 walker derived from climbing ancestors. E, kangaroo, a specialized leaper derived from 

 types like D. F, tree kangaroo, a climber derived from kangaroos. (After D. Dwight Davis, 

 Jr., by permission General Biological Supply House, Inc.) 



plexity which characterize the placental mammals, particularly in the 

 organization of the nervous system. 



The earliest known placentals were small late Cretaceous insectivores 

 related to our modern shrews and moles. They had long skulls with many 

 sharp teeth, small brains, and flexible feet with pointed claws ; they were 

 agile, and may well have been nocturnally active tree dwellers. From such 

 an ancestral stock the placental mammals rapidly developed along many 

 lines during the Paleocene, and by Eocene times the principal directions 

 of their later evolution had become established. 



In all of the groups that arose from the insectivores there has been a 

 tendency toward increase in relative brain size and in size of body, accom- 

 panied by modifications of the skull, teeth, and limbs. In each stock some 

 archaic groups early became specialized and attained temporary domi- 

 nance, only to be replaced later by more advanced modernized types with 

 better brains, that rose from the ranks of the less specialized. Several 

 different groups of early placentals became herbivores, taking advantage 



