TERTIARY MAMMAL HORIZONS. 



49 



fauna according to the laiv of adaptive radiatioii" from primi- 

 tive and central types. Branches will spring off in all directions 

 to take advantage of every possible opportunity of securing food. 

 The modifications which animals undergo in this adaptive radi- 



ANTXRCriC 



Fig. I. — Division of the World into three Realms and nine main Geographical 

 Regions. The continental platform is raised to the 200 metre line showing the 

 main Tertiary land connections. 



ation are largely of mechanical nature, they are limited in num- 

 ber and kind by hereditary, stirp or germinal influences, and 

 thus result in the independent evolution of similar types in 

 widely separated regions under the law of parallelisDi or hoino- 

 plasy. 



Adaptive Radiation of Orders and Families 



This law causes the independent origin not only of similar 

 genera but of similar families and even of similar orders. 

 Nature thus repeats herself upon a vast scale, but the similarity 

 is never complete and exact. When migrations are favored by 

 over-population or geographical changes, a new and severe test 

 of fitness arises by the mingling and competition of the parallel 

 types. 



1 So termed by the writer (Osborn, '93 and '99). 

 Annals N. Y. Acad. Sci.,'XIII, July 19, 1900-4. 



