
No. 425.] LAW OF ADAPTIVE RADIATION. 357 
hypothesis is expressed in Fig. III (Fig. 1]. The other orders of mam- 
mals, the Sirenia (probably a branch of the hoofed tribe), took the rivers 
and coasts of America, Europe, and probably Africa as their radiating 
center, while the Cetacea occupied the fourth or oceanic realm. 
e mean to express by this hypothesis that REALMs [Fig. 2] were the 
main centers of adaptive radiation of orders of mammals, but by no 
ye ARCTIC 
iy 
EURASIATIC NORTH TEMPERATE NORTH 
(Palaearctic) (Hola €) ileyna 
AWC TO CARD ; 

CLE. — IND0-MALAYAN 
Boy (Oriental) 

N R 
DaN Sin, 
(Ethiopian) | . t 
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> A.P > 
MALACASY ^ souT 
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Fic. 2. — Division of the x into three realms and nine main geographical regions. The 
tiene platform is raised to the 200 meter line showing the main Tertiary land connec- 
tions. (From Osborn 
means the exclusive areas of distribution, for during the periods of land 
contact certain members of these orders found their way into adjacent 
realms. Each realm, therefore, contains its pure autochthonous types an 
its migrant or derived types. REGIONS, on the other hand, may be dis- 
tinguished from realms as geographical and zoólogical areas, which have 
been isolated from each other for shorter periods, either by climatic bar- 
riers, as in the case of the Arctic or circumpolar region, or by great physi- 
cal barriers, such as masses of water and of desert sands. In certain cases 
these regions, such as Africa, appear to have been so large, distinct and 
isolated, as to have become important centers of the radiation of certain 
orders of mammals and almost attain the rank of realms, but regions in 
general are chiefly and permanently distinguished by the adaptive radiation 
of families of mammals. 
In this paper Africa was treated hypothetically as a great 
center of independent evolution and as the source of successive 
northward migrations of animals. This hypothesis has recently 
been confirmed by remarkable paleontological discoveries in 
northern Africa. This adds to the above list of five radiations 
a sixth, namely : 
