Introduction. xxvii 



of this river is also true of many other rivers in the dryer portions 

 of Mexico, and especially so of those streams whose waters never 

 reach the sea. 



RECENT GEOLOGICAL CHANGES IN MEXICO AND THEIR 

 GENERAL EFFECT ON THE FISH FAUNA. 



Geologically speaking, within more recent times the climatic and 

 hydrographic conditions of Mexico have been less stable than in the 

 Mississippi Valley. The central portion of Mexico has been subject 

 to considerable volcanic disturbances which have continued to within 

 recent times. The northern portion has evidently at some former 

 time been much better watered than it is now. Such streams as the 

 Rio Casas Grandes, Rio Santa Maria, Rio Carmen, the Rio Nazas, 

 and many others in this region whose waters never reach the sea were, 

 perhaps, formerly tributaries of the Rio Grande. This supposi- 

 tion is based on the character of their fish faunas and the 

 general topography of the country. Dr. O. C. Farrington, of the 

 Department of Geology of this Museum, suggests that the reduced 

 size of these streams is due largely to the fact that most of the moun- 

 tain drainage of this part of Mexico is now to the west; and that the 

 portions of the western rivers which are east of the Sierra Madre 

 Mountains were formerly the upper tributaries of the streams which 

 flow to the east. According to this view the western streams have 

 cut their way back, captured the head -waters of the eastern streams, 

 and with them their portion of the eastern fish fauna. Mr. A. V. 

 Temple, who has traveled extensively over this region for the past 

 thirty-five years, informs me that there is much less water in this por- 

 tion of Mexico now than when he first visited it. Many lakes have 

 become entirely dry, though occasionally one, as La Laguna in Chi- 

 huahua, which has been dry for years, may be partially filled by a heavy 

 rain. When the Mexican Central Railroad built its line from El Paso 

 to the City of Mexico it was extended across the dry bed of this lake. 

 About seven years ago a heavy rain submerged this portion of the 

 track and delayed traffic until the road could lay its present line some 

 distance to one side of the lake. Since then, La Laguna has grad- 

 ually become smaller and is likely soon to return to a dry basin again. 

 Disturbances of the kind noted above have their influence on the 

 aquatic life of the country. Any change in environment means that 

 animals affected by it must change physically to meet the new con- 

 ditions ; those that are unable to change to suit these conditions sooner 

 or later disappear, while new varieties, and, if the time is long enough, 

 new species, come into existence. 



