506 MEMOIRS OF THE CARNEGIE MUSEUM 
epoch had its effect upon the crawfishes living in them. They developed into as 
many species as there were rivers (three). Probably there was already in Tertiary 
times a tendency within the Erigan drainage to form variations and even geograph- 
ical varieties, but the fact that these forms (at least two of them) assumed the char- 
acter of species is due to the physiographical features of the earlier Glacial Period. 
After the Ohio was formed, and the connection between the areas of these 
species was reestablished, there must have been a tendency among them to mix 
along the course of the Ohio River. How it was in the case of C. propinquus and 
C. sanborni we do not know. But I have investigated the facts in the case of C. 
sanborni and C. obscwrus. Where they come together in the neighborhood of New 
Martinsville, West Virginia, C. sanborni shows no tendency at all to go up the river, 
no trace of it being found above New Martinsville. This apparently is due to the 
greater difficulty of ascending the river and to contend with a species which is firmly 
established there. On the other hand C. obscurus apparently has gone down the 
river, and has invaded the original territory of C. sanborni, but it has done so only 
to a small extent. For, although it is easier to descend a river, the fact that the 
region invaded is occupied by another species with the same ecological habits must 
make it rather difficult to oust the latter. Thus, although C. obscwrus has the advan- 
tage over C. sanborni, being favored in its migration by the fact that it is here down- 
stream, this advantage is only a slight one, and did not enable C. obscwrus to occupy 
any of the territory of C. sanborm to the exclusion of the latter. It is found here 
associated with it, but its numbers are small, and the original form still prevails. 
A curious fact, however, has been observed. I have pointed out (p. 367 and p. 
434) that the specimens of C. sanborni captured in Fishing Creek at New Martinsville 
showed in certain characters an inclination toward C. obscurus. This suggests hybrid- 
ization. Of course it is impossible to ascertain this positively without experiments, 
but it seems that a crossing between these two forms is not altogether impossible, 
for the shape of the sexual organs is very similar in both. They are generally very 
closely allied, and further, their breeding seasons are identical, so that kyesame- 
chania® probably does not exist. This is further suggested by the conditions ob- 
served in the Lake Erie drainage in Pennsylvania. Here C. propinquus and C. 
obscurus come together, and again we pointed out (p. 565) that C. propinquus in this 
region has a tendency towards C. obscwrus. In both cases hybridization would 
easily explain matters. 
®6>Impossibility of crossing, due to any cause, mechanical, physiological, or ecological, see Eimer, 1895, p. 14. 
Gulick (1995, p. 95) calls this ‘‘ Impregnational Isolation.’? Under this head falls also Romanes’ ‘‘ physiological 
isolation.”’ 
