ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 511 
(e) C. diogenes. 
C. diogenes is sharply separated from the other chimney-builders, but resembles 
them ecologically to a certain degree. In Pennsylvania it comes into contact with 
them, but in the case of C. carolinus this has been observed only once, while it is 
more frequent in the case of C. monongalensis. However, intermediate forms have 
never been observed, so that we must assume that kyesamechania prevents crossing. 
In both cases, with reference to C. carolinus as well as C. monongalensis, it is to 
be remarked that whenever one of these is found associated with C. diogenes it 
is always only a contact, not a real mixing of both forms. This is best observed in 
the case of C. monongalensis and C. diogenes. All over the range of C. monongalensis 
in southwestern Pennsylvania C. diogenes is also found. But as has been stated 
(p. 417 and 458), although they frequently dwell at the same localities they do not 
occupy the identical locations, C. diogenes belonging to a lower level than C. mononga- 
lensis. ‘Thus we see again a separation, which is primarily expressed in the difference of 
altitude. Whether the latter is most important seems doubtful. It has been stated 
that C. monongalensis prefers spring-water, while C. diogenes lives mostly in swamps, 
where the water is more or less stagnant and not so cool in summer. (Compare the 
instance from Nine-Mile Run given above.) But, whatever may be the essential 
feature which separates both species, it is clear that it is an ecological factor, and, 
when these two species are found together, it is at a place where the ecological con- 
ditions favorable to them come together. 
That C. diogenes depends on different ecological laws from C. monongalensis is 
also evident from the fact that the former has, outside of Pennsylvania, an entirely 
different range. 
Thus we have here a case similar to that of C. bartoni when it associates with the 
burrowing forms. Two allied species occupy (in Pennsylvania) almost the same ter- 
ritory, and are not separated topographically, but their ecological separation is evi-- 
dent, and very likely is connected with their specific differentiation. 
In the two races C. diogenes, the eastern and western, we again see the influence 
of separation. According to our theory that the area of C. diogenes was a unit in 
Preglacial times, and that it was separated by the advancing ice into an eastern and a 
western section, which subsequently remained separate, we must expect, if isolation 
effects specific differentiation, that the eastern and western form of C. diogenes should 
show at least a tendency to develop differential characters of specific value. This is 
indeed the case, as we have seen above (p. 401 et seq). 
Isolation, or Habitudinal Segregation, as the factor forming species, is thus 
clearly seen in every case discussed. We may condense the results obtained in the 
following sentences. 
