BUFFALO SOCIETY OF NATURAL SCIENCES 205 



the former are easily understood for the latter. I do not mean to 

 imply that migration of river forms all around the world can take 

 place always in a single geological period. It is well known that 

 certain related or identical species of fish which today are found in 

 rivers thousands of miles apart have such a distribution because 

 their ancestors in a former geological epoch when relations be- 

 tween land and sea were different had an opportunity to accomplish 

 the migrations, all evidences of which have since been destroyed. 

 In distributions observed today we see the result of migrations which 

 may have taken place ten thousand or ten million years ago. Thus 

 Giinther observes that the present occurrence of the Dipnoi on the 

 continents of Africa, South America and Australia is consequential 

 upon their wide range in the Palaeozoic and Mesozoic, while that of 

 the Siluroids, which have an even greater range, is the result of their 

 distribution during the Cenozoic. It may be well to refer here to 

 the theory of the independent origin of specific characters, in widely 

 dispersed organisms, which are, nevertheless, placed under similar 

 or identical physical conditions. This theory has been especially 

 applied to the fishes of South America by Hasemann (110), who has 

 shown how further complications arise through the production of 

 apparently identical though actually unrelated species in response 

 to similar environmental complexes. 



Summary. Observations upon freshwater fishes have brought 

 out the following facts as to dispersal and migration: 



1. Dispersal and migration take place in circumpolar zones the 

 range of migration depending upon: (a) temperature, (b) climate, (c) 

 euryhalinity or stenohalinity of species, genera, etc., (d) vitality of 

 given individuals to withstand sudden changes in temperature, in 

 salinity, or in the amount of available water and food supply. 



2. The interlacing of the headwaters of mighty river systems of- 

 tentimes accounts for the occurrence in the lower reaches of rivers 

 hundreds of miles apart of identical or closely similar genera and 

 species. The case of the trout on the North American continent is a 

 familiar illustration. In the interlacing headwaters of both the Co- 

 lumbia and Missouri rivers occurs the cut-throat trout, Salmo clarki. 

 Various species are gradually differentiated away from the headwater 

 region. Thus the nearest relatives of S. clarki are S. virginalis in 

 the basin of Utah, and S. sternias of the Platte River. "Next to the 

 latter is Salmo spilurus of the Rio Grande and then Salmo pleuriticus 

 of the Colorado. The latter in turn may be the parent of the Twin 



