512 MEMOIRS OF THE CARNEGIE MUSEUM 



1 . The normal case is when two closely allied species, possessing identical or 

 nearly identical ecological habits occupy separated areas, which lie close together but 

 do not overlap. (Examples : j^i'opinquus-gronp ; C. carolinus and C. nionongalcnsis.) 



2. Whenever allied species are found in one and the same locality (overlapping), 

 isolation becomes apparent in the following forms. 



(a) The two species have different centers of origin, that is to say, they were 

 separated formerly, Init occupied the same territory subsequently. In this case, if 

 very closely allied, hybridization maybe possible (C. o/;sc»n(.s and C. sanborni at 

 New iNIartinsville, and C. ohscurus and C. propinquus in the Lake Erie drainage), 

 if no kyesamechania exists. If the latter is present, which always means that the 

 two species in question are less closely allied, the two species may actually live 

 side by side under identical conditions (C. hartoni and the river-species), or one may 

 conquer and suppress the other. No instances of the latter kind are known in 

 Pennsylvania, but may possibly occur in southwestern Ohio and in Indiana, between 

 C. nisticus and C. propinqims. 



(b) If the centers of origin are more or less identical (absolute identity is hardly 

 possible), the two species always differ ecologically, and although li\ing at the 

 same localities, prefer different surroundings. In this case they are not so closely 

 associated, and they generally remain at a certain distance from one another, 

 although their general areas are overlapping. Under such conditions hybridisation 

 might occur, but it has not been observed in Pennsylvania, and the species existing 

 under such conditions are probably separated by kyesamechania. (Example : C. 

 diogenes and raonongalensis.) 



Case (fl) and [h) may be combined, that is to sa}^, two species living together may 

 have different centers of origin and may be ecologically different. This is seen in 

 the example of G. hartoni and the burrowing species. 



I believe that in every case where closely allied species overlap in parts of their 

 ranges a close investigaticm will reveal that one or the other of the above cases is 

 realized. Isolation is, in my opinion, a necessary factor in the differentiation of 

 species, and I do not think that a case ever will be discovered where two closely 

 allied species possess precisely the same distribution. But in order to ascertain this 

 a rnere superficial knowledge of the species in question and tlieir range is insuffi- 

 cient, and every case should be investigated as exactly as possible, in a manner 

 similar to the. above studies. 



