2/4 SCIENCE SKETCHES. 



substantially alike indicates that the date of crossing 

 from one to the other, say from Denver to Pueblo, 

 is comparatively recent. The runway is not how- 

 ever yet made out, but it probably lies between 

 Pike's Peak and Denver, and may have been due 

 to some glacial overflow from the South Platte 

 into the creek called Font-qui-Bouille. The pas- 

 sage from the Missouri to the Platte is older, for 

 here the trout have become perceptibly changed. 

 The trout of the Platte 1 and Arkansas is small, 

 very green in color, with very red flesh; the spots 

 are gathered chiefly on the tail, and the red cut- 

 throat mark is bright. 



From the Arkansas River to the Rio Grande 

 over the Sangre de Cristo Mountains is again but 

 a step, a short step, but a very higji one. This 

 the trout has in some way crossed. Here again 

 we may imagine glacial lakes now drained as the 

 way of passage, or, still better, we may say we 

 do not know. The transfer must have been an 

 old one, for the trout in the Rio Grande 2 is visibly 

 different, the difference consisting in the larger 

 scales and smaller size of the black spots. Once 

 more across the main divide we follow the trout, 

 from the tributaries of the Rio Grande to those 

 of the Colorado. Here again the point of trans- 

 fer is unknown, and here once more the imagina- 

 tion and the glaciers must fill up the gap. It 

 is not far from Rio Chama over to the Rio San 



1 The "Greenback Trout" of the Arkansas and Platte is 

 Salmo mykiss stomias Cope. 



2 The trout of the Rio Grande is Salmo mykiss spilurus Cope. 

 Its range extends farther southward than any other known form, 

 as far as the mountains of Chihuahua. 



