ORIGIN OF MAMMALS 



235 



is probably a remote descendant of an insectivorous type 

 not very distant from the existing tree shrews (Fig. 112), the 



transformation of size, of func- 

 tion, and of form between these 

 two extremes having taken 

 place within a period broadly 

 estimated in our geologic time 

 scale at about 10,000,000 

 years. 



Fig. 112. The Tree Shrew Tiipaia. 

 Insectivore, considered to be near the pro- 

 totype form of all the higher placental 

 mammals. 



Origin of the Mammals, Insec- 

 tivorous, Arboreal 



To the descent of the mammals 

 Huxley was the first, in essaying the 

 reconstruction of the great ancestral 

 tree, to apply Darwin's principles 

 on a large scale and to prophesy 

 that the very remote ancestral 

 form of all the mammals was of an 

 insectivore type. Subsequent re- 

 search' has all tended in the same 

 direction, pointing to insectivorous 

 habits and in many ways to arboreal 

 modes of existence as characteristic 



Fig. 113. rKiiUiTUE Types of 



MONOTREME AND MARSUPIAL. 



(Below.) Monotreme type — Echid- 

 na, the spiny ant-eater. 



(Above.) Marsupial type — Didel- 

 pliys, the arboreal opossum of 

 South America. After photo- 

 graphs of specimens in the New 

 York Zoological Park. 



* This insectivorous and tree-inhabiting theory of mammalian origin has recently 

 been advocated by Doctor William Diller Matthew of the American Museum of Natural 

 History, by Doctor William K. Gregory of Columbia University ("The Orders of Mam- 

 mals"), and Doctor Elliot Smith of the University of Glasgow. 



