590 



SCIENCE. 



[N. S. Vol. XXI. Xo. 537. 



sels require direct fresh-water communication 

 in order to pass from one stream to another. 

 It appears that the recorded observations of 

 many naturalists and the facts of Unionidse 

 distribution are both contrary to this concep- 

 tion. In the first place, there are so many 

 authentic cases where birds, insects, etc., have 

 been taken with fresh-water shells attached to 

 them, that students of the subject are com- 

 pelled to accept this method of dispersion of 

 these forms from place to place. Darwin 

 proved that young molluscs just hatching will 

 attach themselves to the feet of a duck, and 

 remain alive in this position out of water 

 from twelve to twenty hours. Mr. Arthur F. 

 Gray, of Danversport, Massachusetts, had in 

 his possession the foot of a water fowl to 

 which was attached a bivalve shell. Canon 

 Tristran shot a bird in the Sahara which had 

 attached to it the eggs of some mollusc. Some 

 shells attach themselves to plants which are 

 carried away by birds (Darwin). Insects are 

 frequently taken with shells attached. There 

 are at least five recorded cases of the capture 

 of the water-scorpion, Nepa, a large flying 

 bug, with small shells attached. The great 

 water-beetle, Dytiscus, is known similarly to 

 aid in the dispersion of fresh-water mollusca. 

 The same is true of Dineutes. Mr. Albert P. 

 Morse, of Wellesley, has kindly shown me 

 specimens of these last two forms having at- 

 tached shells. Notonecta has likewise been 

 proved to carry these forms from place to 

 place. Some of these insects are powerful 

 fliers. Darwin records the capture of one of 

 them out at sea, forty-five miles from the 

 nearest land. Beddard, Kew and other stu- 

 dents of zoogeography regard birds and insects 

 as undoubtedly important agents in the dis- 

 persion of fresh-water shells. Woodworth 

 catalogues a number of agencies recorded as 

 aiding in this dispersion, in addition to those 

 mentioned above. It appears, then, that other 

 means besides river-capture for the passing of 

 fresh-water shells from one stream to another 

 are not lacking. That these means are efii- 

 cient is proved by the distribution of these 

 shells. Ponds are sometimes made by ex- 

 cavating a place where no water stands or- 

 dinarily, lining the excavation with concrete 



and allowing the rain to fill it. These ponds, 

 for a time devoid of life, gradually become 

 populated with molluscs and other shells, prov- 

 ing, as Beddard says, the capacity for active 

 or passive migration on the part of the Mol- 

 lusca. Careful and successive observations 

 have proved in some instances the actual time 

 in which a given pond may become populated. 

 R. Ellsworth Call records the presence of a 

 western species of Unio in a small isolated 

 eastern lake, which was located down between 

 high hills, fed by a mountain brook, and abso- 

 lutely foreign to any stream through which 

 the species might have been introduced. 



But the most conclusive objection to accept- 

 ing this evidence as a proof of river-capture 

 is found in the actual distribution of the very 

 shells upon which the argument is based. The 

 genus Pleurohema is found in both the Ten- 

 nessee and Coosa- Alabama basins. In no case 

 are the species in the two basins identical, but 

 only similar. The basis of the argument, 

 then, is similarity of forms. But if mere 

 similarity of forms proves former river con- 

 nection, certainly identity of form should 

 prove it with double force. Accordingly we 

 should not expect to find the same species of 

 Pleurohema in any two rivers of this section 

 whose location is such as to render practically 

 impossible a former connection with each 

 other. An examination of Mr. Simpson's im- 

 portant monograph on the pearly fresh-water 

 mussels shows that Pleurohema similans Lea, 

 is found in Black Warrior and Cahawba 

 rivers, Alabama, and Pine Barren Creek, Es- 

 cambia County, Florida. So far as can be 

 judged from available maps, previovis fresh- 

 water communication between the former and 

 the latter is extremely improbable. Pleuro- 

 hema strodeana Wright, is recorded from Es- 

 cambia River, Florida, and Flint River, 

 Georgia. Any former connection in this 

 case seems impossible. Pleurohema harperi 

 Wright, is recorded from Altamaha and Flint 

 rivers, Georgia, and Suwanee River, Florida. 

 Here, again, connection between either of the 

 former and the latter seems out of the ques- 

 tion. Other cases might be added — indefi- 

 nitely, if we continue with other genera than 



