Page:The Habitat of the Eurypterida.djvu/213

Rh 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 between 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 Günther 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 Palæozoic 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.

. 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 oftentimes 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 Columbia 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