Maech 30, 190G.] 



SCIENCE. 



505 



distinct harriers,^ but, as I have said before, 

 barriers are not always necessary. Professor 

 Allen says (p. 312), that the 'sedentary dis- 

 position of individual animals ' may act as a 

 barrier, and this is, indeed, what I am think- 

 ing of, namely, the tendency of the individual 

 to stick to certain surroundings, to keep close 

 to certain ecological conditions. 



A case among the crawfishes of Pennsyl- 

 vania may serve as an illustration. 



The southwestern corner of Pennsylvania 

 and northern West Virginia, between Chest- 

 nut Eidge to the east, the Kiskiminetas, Alle- 

 gheny and Ohio Rivers to the north, and the 

 Ohio River to the west, contains two burrow- 

 ing crawfishes (chimney builders), namely, 

 Camharus monongalensis Ortm. and Gamharus 

 diogenes Gir. Both are very closely allied, 

 but they are good species, no transitional 

 forms ever having been found. (Weight may 

 be added to this by the statement that I col- 

 lected, of either form, upward of 300 speci- 

 mens, at 53 localities; at eight places both 

 species were found associated.) 



C. monongalensis is restricted to the area as 

 defined above, while 0. diogenes largely goes 

 beyond it. But within the above boundaries 

 both species are found everywhere, often in 

 very close vicinity, so that their ranges, in 

 this section, truly are overlapping. Moreover, 

 I have reason to believe that their centers of 

 origin are in this same region (southwestern 

 Pennsylvania and northern West Virginia), 

 in the physiographical division called the Alle- 

 ghenian Plateau (more specially: between an 

 Old Tertiary and a Late Tertiary base level, 

 the latter lying below the plateau). 



Here we have a case where two closely allied 

 species became differentiated in the same re- 

 gion, their centers of origin being identical, 

 their present areas being largely overlapping, 

 and they actually living often side by side. 



Nevertheless, there is isolation, and indeed 

 complete isolation, but of purely ecological 

 character. Both are burrowers, but C. mon- 

 ongalensis lives in and near springs with pure 

 and cold water, while C. diogenes is a swamp 



^ See C. H. Merriam, Science, February 16, 

 1906, p. 247 ff. 



form, being content with any kind of water, 

 but preferring more or less stagnant water 

 along ditches, streams and in the river bot- 

 toms. While 0. diogenes is not very par- 

 ticular, C. monongalensis is, and the conse- 

 quence is that both species are isolated from 

 one another, the one {monongalensis) occupy- 

 ing places in the neighborhood of springs, the 

 other {diogenes) appearing a little farther 

 down stream. A further consequence is, that 

 both species are separated to a certain degree 

 according to elevation: C. monongalensis is 

 found generally at altitudes between 1,200 

 and 900 feet, while G. diogenes goes down 

 from about 1,000 to 600 feet. 



As has been said, at certain places, both 

 species come into contact, and, generally, they 

 are never far from each other. Thus we can 

 not talk of a ' barrier ' between them, in the 

 ordinary sense of the word. Nevertheless, 

 they are separated, but the separation is 

 brought about by — as Professor Allen puts it 

 — the ' sedentary disposition ' of these animals, 

 that is to say, by their ecological habits, they 

 being restricted to certain ecological condi- 

 tions, and refusing to leave them. It would 

 be an easy thing for G. monongalensis, for 

 instance, to follow down the stream at the 

 spring of which it lives, and to settle in any 

 swampy places farther below. But it does not 

 do so, and, if single individuals are accident- 

 ally swept down, they do not prosper, because 

 they do not find congenial conditions, namely, 

 pure and cold spring water, which, in this 

 case, seems to be essential. 



This is a case of ' ecological (or bionomic) 

 isolation,' where no ' barriers ' in the ordinary 

 sense are present. It seems that such cases 

 are less frequent than topographic or climatic 

 isolation, but I believe more will be discovered, 

 as soon as attention is called to them, and I 

 have no doubt that Professor Allen is able to 

 quote similar cases offhand from his own 

 rich experience. I called, years ago, atten- 

 tion to a case which, indeed, suggested the 

 whole idea to my mind, namely, the case of 

 different species of the decapod genus Uca 

 (fiddler crabs), living closely associated on 

 the coast of East Africa, but each preferring 



