86 SCIENCE SKETCHES. 



misleading, or so numerous as to be confusing; 

 and the few writers who have taken up this subject 

 in detail have found both these difficulties to be 

 serious. Whatever general propositions we may 

 maintain must be stated with the modifying clause 

 of " other things being equal ; " and other things 

 are never quite equal. Dr. Wilder's saying that 

 "■ Nature abhors a generalization " is especially ap- 

 phcable to all discussions of the relations of species 

 to environment. 



Still less satisfactory is our attempt to investi- 

 gate the causes on which our partial generaliza- 

 tions depend, — to attempt to break to pieces the 

 " other things being equal " which baffle us in our 

 search for general laws. Scarcely anything has 

 been written on this phase of the subject from an 

 American point of view. This little I have tried 

 to include with my own observations, in preparing 

 this paper. The same problems, of course, come 

 up on each of the other continents and in all 

 groups of animals or plants ; but most that I 

 shall say will be confined to the question of the 

 dispersion of fishes in the fresh waters of North 

 America. The broader questions of the bounda- 

 ries of faunae and of faunal areas I shall bring up 

 only incidentally. 



Some of the problems to be solved were first 

 noticed by Professor Agassiz in 1850, in his work 

 on Lake Superior. Later (1854), in a paper on 

 the fishes of the Tennessee River,^ he makes the 

 following statement : — 



1 On Fishes from Tennessee River, Alabama. American Jour- 

 nal of Science and Arts, xvii. 2d series, 1854, p. 26. 



