HARRIS : CRAYFISHES GENUS (iAMBARUS. 161 



I think that when proper allowance is made for other factors 

 the suggestion made here of a direct relation between the con- 

 tour of a stream and the number of its species is a helpful one. 



The occurrence of the same species in the head waters of 

 streams on the opposite sides of a watershed has already been 

 referred to, Adams ('01) calls attention to the occurrence of 

 C. sjrinosus, C. erichsonianus and C. exti-aneus in the Tennessee 

 river, and also in the head waters of the Coosa-Alabama sys- 

 tem, and calls attention to its significance. The occurrence 

 of C. carolinus, apparently an upland species, in northeastern 

 Indian Territory, has been noted." Many cases of apparent iso- 

 lation are doubtless due to our imperfect knowledge of distri- 

 bution. In several cases I have found instances in which 

 upland species occurred in the lower portion of the stream un- 

 der conditions which would seem to be quite different from 

 their usual ones. Whether these isolated localities are merely 

 the edge of the range of the species which is to be found under 

 very diverse conditions, or whether its occurrence there is acci- 

 dental, remains to be seen. 



Distribution in its broader sense I shall not consider. Faxon 

 ('85) and Ortmann ('02) have discussed the distribution of the 

 groups and the genus, and have advanced theories for the ori- 

 gin of the genus and its groups. Their theories are somewhat 

 at variance with each other, but space cannot be devoted to a 

 discussion of them here. My object has been to consider the 

 ecological factors, and while some interesting points present 

 themselves in connection with the groups the time is not ripe 

 for their discussion. 



27. lathis place a statement made by Jordan (Science Sketches) is interesting: "Again, 

 streams of the Ozark mountains, similar in character to the rivers of East Tennessee, have an 

 essentially similar fish fauna, although between the Ozarks and the Cumberland range lies an 

 area of lowland bayous into which such fishes are never known to penetrate. We can, how- 

 ever, imagine that these upland fishes may be sometimes swept down from one side or the other 

 into the Mississippi, from which they might ascend oq the other side. But such transfers cer- 

 tainly do not often happen. This is apparent from the fact that the two faunae are not quite 

 identical, and in some cases the same species are represented by perceptibly diffor?nt varieties 

 on one side and the other. The time of the commingling of these fauiife is perhaps now past, 

 and it may have occurred only when the character of the intervening region was culder than it 

 is at present." 



8-Bull., No. 3. 



