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 for ms'( 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, thei-e must have been a tendency among tliem to mix 

 along the course of the Ohio River. How it was in the case of C. jiropinquus and 

 C. sanhorni we do not know. But I have investigated the facts in the case of C. 

 sanborni and (J. (jbscurus. Where they come together in the neighborhood of New 

 Martinsville, West Virginia, C. sanhorni shows no tendenc}' 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 (\ ohscurus apparently has gone down the 

 river, and has invaded the original territory of (1 sanborni, but it has done so only 

 to a small extent. For, although it is easier to descend a liver, 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. ohscurus 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. obscmms to occupy 

 any of the territory of C. sanborni 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. 

 4.34) that the specimens of C. sanborni captui-ed 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. lyropinquus and C. 

 obsc^irtis come together, and again we pointed out (p. 365) that C. propi7iquus in this 

 region has a tendency towards C. obscurus. In both cases hybridization would 

 easily explain matters. 



** Impossibility of crossing, due to anj' cause, mechanical, physiological, or ecological, see Eimer, 1895, p. H. 

 Gulick (1905, p. 95) calls this " Impregnational Isolation." Under this head falls also Romanes' "physiological 

 isolation." 



