MATTHEW, CLIMATE AND EVOLUTION 297 



Dispersal of Fresh-water Fishes 



The fresh-water fishes afford many striking illustrations of isolated 

 primitive survivals in the southern continents and especially in their 

 tropical parts. With marine fishes, the distribution is wider, as we should 

 expect, and the dominant types are generally world-wide in their distri- 

 bution. Yet, even with marine fishes, a superficial survey seems to show 

 the majority of primitive survivals along the southern coasts. 



Fishes are, it is to be remembered, dominantly marine. The wider 

 field and more varied opportunities for development afforded by the 

 ocean waters, in contrast with the limited and isolated fields and uncer- 

 tain tenure afforded by fresh-water rivers and lakes, have conditioned 

 this. The fresh-water habitat for aquatic groups of animals stands in 

 somewhat the same relative position to the marine habitat as does the 

 insular to the continental habitat for land animals. It is the refuge for 

 survivors of primitive faunas. And, as in the insular land faunae, we are 

 constantly confronted there with the occurrence in widely remote regions 

 of archaic types apparently nearly related, whose similarity is partly due 

 to independent adaptation to a similar environment, partly to persistent 

 primitivism. 



Lepidosiren in tropical South America, Protopterus in tropical Africa, 

 Ceratodus in tropical Australia are perhaps the most prominent examples 

 of extremely ancient survivals. These are survivors of early Mesozoic or 

 even Paleozoic marine and estuarine fishes of world-wide distribution, 

 and they have endured, in their tropical refuge, the several successive 

 periods of zonal climate which affected the environment of temperate and 

 tropical regions. 



More pertinent to the problem in hand are the relationships of early 

 Tertiary fishes of the northern continents to the modern South American, 

 African (and Australian?) fishes. Here, again, I am compelled to dis- 

 sent from the interpretations and conclusions of so distinguished an au- 

 thority as Dr. Eigenmann,^^^ who, as it seems to me ignores certain very 

 important parts of the evidence. 



There is a marked similarity between certain parts of the fresh-water 

 fish faunge of South America and of Africa. Eigenmann and others 

 would explain this by a former continental union, l)ut it is certain that 

 some, at least, of these now tropical types existed in the northern conti- 

 nents during the early Tertiary. Eigenmann^^'' asserts, indeed, that no 



lis gee especially C. H. Eigen.manx : "Fresh-water Fishes of Patagonia," Reports 

 Princ. Univ. Exped. Patasconia, vol. iii. parts iii-iv. 1000-10. 

 ""C. H. EiGEX.M.wx : Popular Science Monthly, 1906, p. 523. 



