ETHEOSTOMIN^E THE DARTERS 279 



discussed, the darters thus form a rather definite ecological assem- 

 blage, assimilated by their like adaptive characters and by their 

 similar relations to like situations. There are, nevertheless, well- 

 marked degrees of adaptation among the different genera and 

 species ; and, likewise, in the strictness of their confinement to the 

 class of situations characteristic of the group. Three of our species, 

 for example, are often found in still or sluggish waters and over a 

 muddy bottom; one, the sand-darter, is much the commonest in 

 streams with a sandy bottom ; and another, Cottogaster shumardi, is 

 most abundant along the borders of the largest rivers. The species 

 are likewise distinguishable in other features of their local distribu- 

 tion, as may be readily seen by a comparison of the distribution 

 maps of the darters appended to this report. The force of competi- 

 tion is thus more or less broken among them in various ways, no 

 exact analysis of which has ever been attempted. The origin of 

 these species is an interesting and inviting problem, particularly 

 open to solution because of the comparatively restricted range of the 

 -family and the fact that there is nothing to suggest an extensive 

 migration from the place of their original differentiation. 



The food of the subfamily was studied by the senior author many 

 years ago from the contents of seventy stonrachs representing fifteen 

 species, collected in various parts of Illinois in several months of four 

 successive years. These indicated more than their number would 

 imply, since different darters obtained from the same locality and on 

 the same day usually agreed so closely in food that the study of from 

 two to five specimens gave all the facts obtainable from several 

 times as many. Furthermore, the differences between the related 

 species in respect to food are so slight that specific peculiarities were 

 scarcely recognizable. The data obtained, therefore, really apply to 

 /the food of the whole subfamily at different seasons in twenty-nine 

 •localities within this state. This was, on the whole, remarkably uni- 

 form, except that two of the species, the largest and the smallest of 

 the group, were found to differ from the remainder in a way to cor- 

 respond to a notable difference in their local distribution. 



Briefly described, the typical species feed on insect larvas com- 

 monly abundant on the bottoms of streams/under or among stones, 

 ^nd in other similar situations, the smaller species eating mainly 

 dipterous larvae (most commonly Chironomus) , together with a 

 smaller proportion of neuropterous larvas of the smaller sizes ; while 

 the larger darters eat essentially the same kinds of food, but in dif- 

 ferent ratios, the neuropterous larvae being of larger average size and 

 also making a larger part of the food. The two exceptional species 



