GEOGRAPHIC ISOLATION 79 



part of outward appearance, but may include details of structure, 

 such as the form of the teeth, and is sometimes so great that one may 

 speak of convergence. Thus the resemblance between the mole-like 

 marsupial (Notoryctes) of Australia and the placental golden mole 

 (Chrysochloris) of Africa, due to their common adaptations for bur- 

 rowing, is so great that Cope assumed a direct relationship between 



the two forms. 15 



Examples of adaptive radiation which produces ecological equiva- 

 lents are numerous. Closely allied vicarious forms are likely to inhabit 

 the same ecological niche; ecological equivalence, however, may 

 properly be extended to include the presence of taxonomically unre- 

 lated animals, in remote parts of the world, but in similar habitats, 

 as in the moles just cited. Such forms as the plains-dwelling kangaroos 

 may be regarded in this sense as ecological equivalents of the horses, 

 though without morphological resemblance. The characins form about 

 a third of all the fresh-water fishes of South America, and this single 

 family has representatives of all types from the herbivorous forms to 

 the predatory ones, corresponding to our carp, whitefish, trout, pike, 

 and catfish, which belong to as many different families. Among the 

 South American birds the ovenbird family (Furnariidae) shows a 

 similar adaptive radiation, replacing birds of the most diverse families 

 in other parts of the world; they live on the ground, burrow in it, 

 live in bushes, or climb like woodpeckers; they inhabit the rain-forests 

 and hot dry valleys, the banks of mountain streams, or the pampas; 

 they reach the high mountains, and small Synallaxis are found in the 

 most barren parts of Patagonia; they feed on seeds, or insects, or 

 worms and snails; they are correspondingly varied in appearance, and 

 differ in size, in the form and size of bill, in length of limb and toes, 

 and in the length and form of the tail. Fifty-five genera of this family 

 are now recognized, with hundreds of species. 16 



The auks and penguins are analogous groups in the arctic and 

 antarctic seas. The extinct great auk, the dovekie {Alle alle) and 

 razor-billed auk (Alca tor da) are the northern forms, the emperor 

 and adelie penguins and their relatives, the southern. They resemble 

 each other greatly in habits, somewhat less in general appearance. 



Bodies of water isolated for long periods may acquire a similarly 

 radiating fauna. The crustacean suborder Cumacea is represented in 

 the Caspian Sea by the single genus Pseudocuma, which is rare in the 

 ocean, while in the Caspian it has thirteen very diverse species. These 

 are especially remarkable in paralleling other distinct genera of the 

 oceanic Cumacea; thus, among other examples: Pseudocuma diasty- 



