Dispersion of Fresh- water Fishes 295 



In our fresh-water fishes each species on an average has been 

 described as new from three to four times, on account of minor 

 variations, real or supposed. In Europe, where the fishes have 

 been studied longer and by more different men, upwards of six 

 or eight nominal species have been described for each one that 

 is now considered distinct. 



Special Creation Impossible. — It is evident, from these and 

 other facts, that the idea of a separate creation for each species 

 of fishes in each river-basin, as entertained by Agassiz, is wholly 

 incompatible with our present knowledge of the specific dis- 

 tinctions or of the geographical distribution of fishes. This is 

 an unbroken gradation in the variations from the least to the 

 greatest, — from the peculiarities of the individual, through local 

 varieties, geographical sub-species, species, sub-genera, genera, 

 families, super-families, and so on, tmtil all fish-like vertebrates 

 are included in a single bond of union. 



Origin of American Species of Fishes. — It is, however, evi- 

 dent that not all ^Vmerican types of fishes had their origin in 

 Amicrica, or even first assumed in America their present forms. 

 Some of these are perhaps immigrants from northern Asia, 

 where they still have their nearest relatives. Still others are 

 evidently modified importations from the sea; and of these 

 some are very recent immigrants, landlocked species which have 

 changed very little from the parent stock. 



The problems of analogous variation or parallelism without 

 hom^ology are very often met with among fishes. In shallow, 

 swift brooks in all lands there are found small fishes which hug 

 the bottom — large-finned, swift of movement, with speckled 

 coloration, and with the air-bladder reduced in size. In the 

 eastern United States these fishes are darters, dwarf perches ; in 

 northern India they are catfishes; in Japan, gobies or loaches; in 

 Canada, sculpins; in South America, characins. Members of 

 various groups may be modified to meet the same conditions 

 of life. Being modified to look alike, the thought of mutual 

 affinity is naturally suggested, but in such cases the likeness is 

 chiefly external. The internal organs show little trace of such 

 modifications. The inside of an animal tells what it really is, 

 the outside where it has been. In other words, it is the ex- 

 ternal characters which are most readily affected by the environ- 



