THE HISTORY OF OUB SPECIES. 87 



of the Tertiary period — at least 3,000,000 years ago. 

 There we find the great sub-class of the placentals, 

 which to-day comprises more than 2,500 species, 

 represented by only a small number of little, insignifi- 

 cant " pro-placentals "; and in these proclwriata the 

 characters of the four divergent legions are so inter- 

 mingled and toned down that we cannot in reason 

 do other than consider them as the precursors of 

 those features. The oldest carnassia (the ictopsales) , 

 the oldest rodentia (the esthony chides), the oldest 

 ungulata (the condylarthrales), and the oldest primates 

 (the lemurarales), all have the same fundamental 

 skeletal structure, and the same typical dentition of 

 the primitive placentals, consisting of forty-four teeth 

 (three incisors, one canine, four premolars, and three 

 molars in each half of the jaw) ; all are characterised 

 by the small size and the imperfect structure of the 

 brain (especially of its chief part, the cortex, which 

 does not become a true "organ of thought" until 

 later on in the Miocene and Pliocene representatives) ; 

 they have all short legs and five-toed, flat-soled feet 

 {plantigrada) . In many cases among these oldest 

 placentals of the Eocene period it was very difficult to 

 say at first whether they should be classed with the 

 carnassia, rodentia, ungulata, or primates ; so very 

 closely, even to confusion, do these four groups of the 

 placentals, which diverge so widely afterwards, approach 

 each other at that time. Their common origin from a 

 single ancestral group follows incontestably. These 

 proclwriata lived in the preceding Cretaceous period 

 (more than 3,000,000 years ago), and were probably 

 developed in the Jurassic period from a group of insec- 

 tivorous marsupials (amphitheria) by the formation of a 

 -primitive j)lacenta diffusa, a placenta of the simplest type. 



