INTRODUCTION 29 



This classification is followed in tho same order in the Appendix of 

 this volume, where the families and principal genera are also introduced. 



Adaptive Radiation and Geography 



We may now consider the relation between the adaptive radiation of 

 the different kinds of manmials and the gcographj^ of past and present 

 times. 



Adaptive radiation, continental} — The law of adaptive radiation natu- 

 rally operates on a grand scale on great continents like Africa, or a great 

 insular continent like Australia. Through its geographic distribution 

 and isolation there originate from common primitive forms new species, 

 genera, families, and even orders of mammals. It is most important to 

 grasp in imagination the ideas of adaptive radiation as applying wherever 

 there is a large theater for the operation of this law and of a succession of 

 radiations in the course of the Age of Mammals. Thus we shall study the 

 proofs of primeval or archaic radiation of mammals which began during 

 the Age of Reptiles and extended in all directions into forms resembling 

 modern insectivores, rodents, bears, dogs, cats, monkeys, sloths, buno- 

 dont, and selenodont hoofed mammals and lophodont hoofed mammals. 

 Through the extinction of many of these mammalian branches or radii, 

 through the survival of other branches, or through the invasion or 

 entrance of branches from some distant radiation, the process begins 

 over again. 



For these grand continental radiations there seems to be some ratio 

 between the degree or extent of divergence and the physiographic diversity 

 and extent of the geographic area in which the radiation occurs. As 

 shown below (p. 38) this connects adaptive radiation with the science 

 of zoogeography or geographic distribution. 



Thus the highly diversified land area of Arctogsea, comprising Africa, 

 Eurasia, and North America, constituted a vast center in which twenty- 

 one primitive and specialized orders of mammals radiated from each other. 

 In the more restricted continent of South America four to five orders of 

 mammals enjoyed their chief radiation. 



Adaptive radiation, local? — Quite as important although not on so grand 

 a scale is the local adaptive radiation in the same or neighboring geographic 

 regions wherever there is found a diversity of habitat and of food supply. 

 Good living illustrations of this local adaptive radiation are seen in the geo-» 

 graphic distriljution in Africa, previous to the extinction by man, of the 

 ''white" or square-lipped rhinoceros [Rhinoceros simus), which lives upon 

 grasses and has long-crowned or hypsodont teeth, and the "black" or 



' See Osborn, Correlation between Tertiary Mammal Horizons of Europe and North 

 America: An Introduction to the More Exact Investigation of Tertiary Zoogeo.ccraphy. Ann. 

 N.Y. Acad. Set., Vol. XIII, no. 1, .July 21, 1900, pp. 49 ff. 



- Osborn, 1902, op. cit. 



