285 



when I say that 63 per cent, of our collections of Hadropterus 

 phoxocepkalus are from rivers and 26 per cent, from creeks; or 

 that 94 per cent, of them are from waters with a bottom of rock 

 and sand and only six per cent, from mud; this means that 

 if miscellaneous collections of fishes of all descriptions had been 

 made from all kinds of Illinois waters until one hundred of 

 them contained darters of this species, then sixty-three of the 

 hundred would have come from rivers and twenty-six of them 

 from creeks, ninety-four of them from rock and sand, and six 

 of them from mud. 



The ratios of this table differ in significance from those 

 of my tables of associative coefficients in the fact that while 

 the latter exhibit various degrees of associative relationship be- 

 tween species, the former express the tendencies or preferences 

 of the species with respect to the features of the physical envi- 

 ronment. An understanding of these physical relations of a 

 species must help us to understand and explain its associative 

 relations, and the one set of data may be expected to serve as a 

 test of the completeness and correctness of the other. 



The Darters as an Ecological Group. 

 It is well known that the darters as a group are most likely 

 to be found in comparatively swift and rocky streams, and that 

 they are especially adapted, by their small size, their large paired 

 fins, their pointed heads, and their habit of resting on the bot- 

 tom, for maintaining themselves in swift currents, and for secur- 

 ing from among and under stones the insect larvae and crusta- 

 ceans on which they mainly depend for food: This fact is clear- 

 ly reflected in my Table VI., of "Local Preferences of Darters", 

 from which it appears that 70 percent, of our collections of the 

 thirteen species were obtained from the smaller streams, 77 per 

 cent, from swift waters, and 82 per cent, from waters with a bot- 

 tom of rock and sand. Only 12 per cent., in fact, came from lakes 

 and ponds, and 18 pet* cent, from waters with a muddy bottom. 



The Typical and the Non-typical Species. 



A comparison, in respect to the strength of their local pref- 

 erences, between the six species which, by means of an analy- 



