ile 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: propinquus-group ; C. carolinus and C. monongalensis.) 
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, but occupied the same territory subsequently. In this case, if 
very closely allied, hybridization may be possible (C. obscurus and C. sanborni at 
New Martinsville, and C. obscurws 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. bartoni 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. rusticus and C. propinquus. 
(b) If the centers of origin are more or less identical (absolute identity is hardly 
possible), the two species always differ ecologically, and although living at the 
same localities, prefer different surroundings. In this case they are not so closely 
associated, and they generally remain ata 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 monongalensis.) 
Case (a) and (b) may be combined, that is to say, two species living together may 
have different centers of origin and may be ecologically different. This is seen in 
the example of C. bartoni and the burrowing species. 
I believe that in every case where closely allied species overlap in parts of their 
ranges a close investigation 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 mere superficial knowledge of the species in question and their range is insuffi- 
cient, and every case should be investigated as exactly as possible, in a manner 
similar to the above studies. 
