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. ©. bartoni 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 bottom 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, topographical isolation is not observed here, but the isolation 1s ¢co- 
logical, and the differentiation of the chimney-builders from C. bartoni very likely 
is connected with and largely due to the latter. 
(d) C. carolinus and C. 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 observed in the western river-species (prop- 
inquus-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. 
W hat the essential feature of this barrier is, is hard tosay. 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 fact 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- 
prngu USs-Zroup. 
