THE AMERICAN NATURALIST. [Vol. XXXVII. 



short time after it, there is a luxuriant growth of vegetation : but 

 after a few months of exposure to the piercing rays of a tropical 

 sun the character of the country changes, and it assumes the 

 air of a parched desert. It is subject to a short rainy season 

 and a long dry one. The rivers which are large in the rainy 

 season become almost dry by the end of the long dry season. 

 Many of the lakes in northern Mexico become dry and the 

 streams which flow into them contain but little water except in 

 the upper part of their courses where they are fed by mountain 



fish. 



like this is very interesting, but no group of living things pre- 

 sents a more interesting subject for the study of geographical 

 distribution than the fresh water fishes. Living as they do 



and lakes and so their dispersion is largely governed by the for- 

 mation of our fresh water lakes and rivers and is therefore 



The two large rivers which reach Mexico from the north and 

 which have furnished highways by which Northern Mexico 

 became stocked with fishes are the Colorado and the Rio 

 Grande. The former flows into the Gulf of California, the lat- 

 ter into the Gulf of Mexico. In their upper courses these two 

 rivers are near each other, but their fishes are not the same. 

 The only fish common to both river basins is a small dace 

 (Rhinichthys dulcis) and this is also found in the head witters of 

 the Arkansas, the Missouri and the Columbia rivers. From the 



