356 THE AMERICAN NATURALIST. [Vor. XXXVI. 
and kind by hereditary, stirp, or germinal influences, and thus result in 
the independent evolution of similar types in widely separated regions 
under the Zaw of parallelism or homoplasy: 
II. ADAPTIVE RADIATION of ORDERS AND FAMILIES AS BEAR- 
ING ON GEOGRAPHICAL DISTRIBUTION. 
This law causes the independent origin not only of similar genera but 
of similar families and even of similar orders. Nature thus repeats herself 
on a vast scale, but the similarity is never complete or exact. n 
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. 
Under the operation of these laws a most interesting generalization or 
hypothesis can be made as to the three [zoólogical] realms : geographical 
ded 
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aI) 






Fic. 1. — Ord f 

during the Tertiary period. (From Osborn.) 
isolation has been so continuous and prolonged that great orders of mam- 
mals have been evolved . . . in each. Thus Arctogea, containing the 
broadest and most highly diversified land area, appears hypothetically as 
the center in which fourteen primitive and specialized orders radiated from 
each other. In the southern portion of /Veogea at least four orders sprang 
from primitive members of the above orders, and the Hystricomorph 
rodents enjoyed their chief radiation. In Mofogea two orders were cut off 
by the sea; one of them a rapidly declining type, the Monotremes, the 
other, the Marsupials, enjoying a very highly diversified radiation. This 
* At this time the distinction between homoplasy and parallelism was not 
appreciated by the writer. 
