THE PLIOCENE PERIOD. 321 



do not altogether tally with this conception, and suggest rather that 

 this was but one of the oscillations of climate that must now be recog- 

 nized as marking geologic history. That the climate was becoming 

 differentiated, and on the whole cooler than it had been in the earlier 

 Tertiary periods, is clearly indicated. 



The land animals. — The history of the mammals continued to be 

 the one great center of interest. Three important features characterized 

 it: (1) A notable intermigration of the continental faunas, including 

 those of North and South America, (2) the initiation later of the present 

 divergence between Old and New World types, and (3) the culmina- 

 tion and perhaps initial decline of the evolution of the placentals, the 

 human and domestic species aside. 



The accelerated intermigration of the early part of the period was 

 a natural consequence of the extension of the land connections brought 

 about by deformative movements. The precise nature of these land 

 connections has not yet been worked out in all the details necessary 

 to a satisfactory interpretation of the biological events of the period. 

 There are outstanding problems as to the extent and continuity of 

 the the connections between Eurasia and America at the northwest 

 and at the northeast, but the evidence of good migratory routes for 

 the land animals, during a portion of the period at least, may be accepted 

 as conclusive. There are also strong hints of the progressive develop- 

 ment of a selective bridge-and-barrier which afforded free passage 

 for some species and shut off others, and this is assignable to increasing 

 cold in the later stages of the period, leading up to the glacial period 

 which followed. This was probably the chief influence in developing 

 the divergence between the mammals of the Old and the New Worlds, 

 for this divergence affects mainly the warm-latitude species. 



The connection between North and South America introduced a 

 biological movement of dramatic interest. There appears to have 

 been no effective isthmian thoroughfare for land animals between 

 the earliest Eocene and the Pliocene or thereabouts, when a way was 

 opened. During the time of the Eocene connection a few mammalian 

 types seem to have sent representatives into South America, and these 

 had evolved on distinctive lines in the interval. A very remarkable 

 group of sloths, armadillos, and ant-eaters had developed from an 

 edentate stem: strange hoofed animals of orders unknown elsewhere 

 (Typotheria, Toxodontia, Litopterna) had arisen from some very primi- 



