284 Dispersion of Fresh-water Fishes 



Generalizations as to Dispersion. — Since those days I have 

 been a fisherman in many waters, — not an angler exactly, but 

 one who fishes for fish, and to whose net nothing large or small 

 ever comes amiss ; and wherever I go I find cases like this. 



We do not know all the fishes of America }'et, nor all those 

 well that we know by sight ; stiU this knowledge will come with 

 time and patience, and to procure it is a comparatively easy 

 task. It is also easy to ascertain the more common inhabitants 

 of any given stream. It is difficult, however, to obtain nega- 

 tive results which are really results. You cannot often say 

 that a species does not live in a certain stream. You can 

 only affirm that you have not yet found it there, and you can 

 rarely fish in any stream so long that you can find nothmg 

 that you have not taken before. Still more difficult is it to 

 gather the results of scattered observations into general state- 

 ments regarding the distribution of fishes. The facts may be 

 so few as to be 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. AVhatever 

 general propositions we may maintain must be stated with the 

 modifying clause of " other things being equal" ; and other things 

 are never quite equal. The saying that "Nature abhors a 

 generalization" is especially applicable to all discussions of the 

 relations of species to environment. 



Still less satisfactory is our attempt to investigate the causes 

 on which our partial generalizations depend, — to attempt to 

 break to pieces the "other things being equal" which baffle us 

 in our search for general laws. 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 boundaries of 

 faunas and of faunal areas I shall bring up only incidentally. 



Questions Raised by Agassiz.— Some of the problems to be 

 solved were first noticed by Prof. 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: 



* On Fishes from Tennessee River, Alabama. American Journal of Science 

 and Arts, xvii., 2d series, 1854, p. 26. 



