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- 
plicable 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 faunz 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. 
