ORTMANN: THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 507 
Even if it should be the case that C. obscwrus may cross with C. propinquus and 
C. propinquus sanborni, this does not invalidate its standing as a species, for we have 
numerous examples in nature in which true species form hybrids. 
Thus we see that these three species, the origin of which as species belongs to 
the beginning of the Glacial time, have come together again; but each seems to 
remain in its original area, and where they come into actual contact the one species 
is hardly able to oust the other. To a small degree hybridization seems to be pos- 
sible at the points of contact. The assumption that C. sanborni might be a hybrid 
between C. propinquus and C. obscwrus is rendered impossible by the exclusive 
presence of C. sanborni all over its range (excepting Fishing Creek), without any 
trace of the two other species. 
It remains to consider the question what the relation of the specific characters 
to isolation may be. We see that in the case of C. obscurus it is chiefly the “shoulder” 
of the male sexual organ which distinguishes this species. This shoulder is found 
at a place where an external stimulus acts upon this organ, namely, just where it 
is touched by the fifth pereiopod in the act of copulation. A similar shoulder is 
found in many other species of Cambarus of different groups and even subgenera, 
and thus it is highly probable that it is this external stimulus which induces the 
development of this feature. But this does not afford us an explanation why this 
shoulder did not develop in other species, especially in C. propinquus. At present 
I am unable to answer this question. The fact remains that we have to deal with 
a specific character, which is clearly due to an external stimulus,” and I have 
always held the opinion that every variation is invariably caused by a reaction of 
the organism to some external influence. (See Ortmann, 1896, p. 188, and 1898, 
p- 157.) But the view that acquired characters are transmissible is not fashionable, 
although now admitted by its chief adversary, Weismann. In consequence of the 
modern tendeney to deny the effect of external causes upon variation, at any rate 
to deny the possibility of the hereditary transmission of such variations, not much 
attention has been paid to the mutual relations between external stimuli and the 
reaction of the organism upon them. But here I think much room for investigation 
is left. In the present case the reaction of the organism upon the external stimulus 
caused by the contact of the fifth pereiopod with the sexual organ is to form at the 
point of contact a notch or angle (shoulder) on the sexual organ. 
This reaction may be slightly advantageous, but it is not absolutely necessary, 
for we see that there are many other species in which this reaction has not taken 
place, even among the most closely allied forms, which are nevertheless well off and 
8i Under ‘‘ pressure of the environment,’’ as Merriam puts it (1906, p. 244). 
