281 



zonule as for Boleichthys fusiformis and Boleosoma camurum. On 

 the other hand,we find no species in which the average coefficient 

 of association is less than 1 — no indication that any of these 

 twelve species are wholly drawn away from their family by 

 stronger ecological affiliations with some other group. Nor do we 

 find, in passing from the more strongly associated species to those 

 less strongly associated, any abrupt transition in the series — a 

 fact which may be taken as evidence that the darters of my list 

 are a unitary group, of which certain species are ecologically 

 more typical than others, having, that is, the darter habits and 

 relationships more fully developed and more strongly fixed. 



Typical and Non-typical Darters. 



The more typical species of this list seem to be the fol- 

 lowing six, mentioned in the order of the size of their coeffi- 

 cients of association: Hadropterus phoxocephalus, Etheostoma 

 zonale, Etheostoma flabellare, Hadropterus aspro, Ammocrypta pel- 

 lucida, and Etheostoma coeruleum, the associative coefficients of 

 which average 2.48. Apparently the least stringently connected 

 with their kind by the associative relation are Diplesion blen- 

 nioides, Etheostoma jessict, Boleosoma earn arum, and Boleichthys 

 fusiformis, the average coefficient of which is 1.36. 



Furthermore, those least strictly associated with darters in 

 general are not especially strongly associated with each other. 

 Of the four species just mentioned, six pairs may of course be 

 made, and the average of the coefficients of these six pairs is 

 1.33 — less by .69 than the general average for the entire group 

 (see Table 111.). If we similarly pair the six species which I 

 have selected as most typical, and average the fifteen coeffi- 

 cients of these pairs (see Table IV.), we get a general coefficient 

 of 3.47 — more by 1.5 than the average for the group. That is, 

 those species which are laxly associated with the darters in 

 general, are also laxly associated with each other; while those 

 which are strongly associated with darters in general, are still 

 more strongly associated among themselves. This last fact was 

 to be anticipated, since in making up the special average coeffi 



