ORTMAXN : THE CRAWFISHES OF THE STATE OF PENNSYLVANIA 507 



Even if it should be the case that C. ohsciinis may cross with C. propinquns 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 orighial 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 p(js- 

 sible at the points of contact. The assumption that C. sanborni might be a hybrid 

 between C. propinquus and C. obscurus 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 cliiefly tlie "shoulder" 

 of the male sexual organ which distinguishes this species. This shoulder is found 

 at a place where an external stimulus a(;ts upon this organ, namely, just where it 

 is touched by the fifth poreiopod in the act of copulation. A .similar shoulder is 

 found in many other .species of Cambarus of difierent groups and even sul)genera, 

 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 witli 

 a specific character, wliicli 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 tendency to deny the effect of external causes upon variation, at any rate 

 to deny the possibility of the hereditary transmission of sucii variations, not much 

 attention has been paid to the mutual relations between external stimuli and the 

 reaction of the organism upon them. P>ut liere 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 tlie sexual urgan is to form at the 

 point of contact a notch or angle (shoulder) on the sexual organ. 



This reaction may be slightly advantagecnis, 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, wliicli are nevertheless well off and 



s'Under "pressure of the environraeDt," aa Slcrriam pata it (1906, p. 24-I). 



