510 MEMOIRS OF THE CARNEGIE MUSEUM 



swampy. At the upper end of the talus, in the outcrop of sandstone rock, and not 

 far (about fifteen feet) from the bed of the spring, is a copious spring, the water of 

 which runs directly into the clay and humus of the pile of talus, in a large part 

 underground. G. hartoni is found in the small stream under stones ; C. monon- 

 galensis is found at and immediately below the spring referred to ; and C. diogenes 

 is abundant all over the pile of talus down to the l;)ottom of the valley. At the 

 upper end of the pile of talus is the place where all three species come close to- 

 gether, but each is subject to different ecological conditions. 



Similar conditions have been frequently observed, and we thus have here the 

 occupation of the same localities by closely allied species, which differ ecologically, 

 that is to say, fopographical isolation is not observed here, but the isolation is eco- 

 logical, and the differentiation of the chimney-builders from C. hartoni very likely 

 is connected with and largely due to the latter. 



( d) C. carolinus and. G. monongalensis. 



We have seen that these two species are very closely allied, but that the distin- 

 guishing characters are constant. Ecologically they are similar, so that hybridisa- 

 tion might occur when they come together. The latter case, however, has never 

 been observed, at least in Pennsylvania, Maryland, and northern West Virginia. 

 The western escarpment of the Chestnut Ridge forms a sharp boundary between 

 them. This case corresponds to that obsei-ved in the western river-species (prop- 

 iriquus-group). Two species identical in their ecological habits are separated topo- 

 graphically. But in this case the barrier separating them is of a different character. 

 What the essential feature of this barrier is, is hard to say. Chestnut Ridge in many 

 respects forms a boundary. Altitude seems to play a part, but whether it is para- 

 mount is doubtful. Absence of extensive deposits of clay on the western side of 

 this ridge on account of the destruction of the Old Tertiary base-level by subsequent 

 erosion, may also be of importance. Further studies in West Virginia surely will 

 lead to a solution of the question, but this much is certain, that these two species 

 again illustrate the rule that " closely allied species occupy neighboring areas," and 

 further they illustrate the ftict that specific differentiation is due to isolation, which 

 is topographical in this case. 



What are the actual causes of the difference of the specific characters (color, 

 shape of rostrum, and sculpture of chelipeds), that is to say, what external influ- 

 ences are responsible for them is even more obscure, as it is in the case of the pro- 

 pinqxms-gr oup. 



