289 



very high, coefficients from Table II., and look up the facts 

 concerning the species compared, as given in Table VI. and in 

 the distrilintion maps appended to this [jajier. Thu.s far, it may 

 lie noticed, I have dealt with aggregates and average numbers 

 only, which, owing to the heterogeneous and variable character 

 of the data, are much more likely to be uniformly reliable than 

 are the separate entries of the tables. The present discu.ssion 

 will, however, necessarily bring into conipari-son these separate 

 entries, and the reasonableness and consistency of the conclu- 

 sions reached by it may serve as some measure of the validity 

 of their individual coefficients. 



By reference to Table II. it will be seen that zeros appear 

 at hve points, in place of coefficients of association an indi- 

 cation that re])resentatives of the several pairs of species con- 

 cerned have never been taken together by us in the same col- 

 lection. This, as already pointed out, must mean either a com- 

 plete difference in general distribution, so far as represented by 

 my collections, or a very radical difference in locality preference. 



Species 1443 and 1461 [Diploiion b/eiinlolfli's and Etlwo^itoina 

 zonnli') are examples. A glance at the distribution maps of 

 these species will show that each has been taken by us only in a 

 different part of the state from the other, hlennioiilcs being con- 

 fined to the Wabash valley, with the exception of a single col- 

 lection at Chicago, and zona/e being limited to the Illinois and 

 Kock river sy.stems. It seems difficult to believe that the flat 

 and indehnite watershed separating the tributaries of the Wa- 

 bash from tho.se of the Illinois, can constitute a physical barrier 

 sufficient to prevent the intermingling of these two species. 

 On the other hand, it mu.st Ite admitted that their ecological 

 relations, as expressed in their prefei'ences of situation, are, on 

 the whole, very similar, as may be seen by a comparison of the 

 two in Table VI. 



A similar explanation is to be made in the case of J)ip/e- 

 ■ilon hlennioidcs and CoUo()iiHti'r sliKniardi (1443 and 1436). Here 

 the areas of our collections of these two species are entirely 

 separate, with the exception of a single collection of Coffofjastcr 

 from the Wabash valley— to which iJiplc-'^ion was entirely con- 



