102 THE POPULAR SCIENCE MONTHLY. 



is dependent on (a) fresh-water communication ; on {JS) character of 

 stream, that is, of water, as to purity, depth, rai:)idity, vegetable 

 growth, etc. ; on (c) the character of the river-bed, as to size, condi- 

 tion of bottom, etc. ; on {cV) climate, as determined by latitude and 

 by elevation above the sea ; and, finally, on (e) various unknown 

 factors arising from the nature or the past history of the species in 

 question, or from the geological history of the rivers." 



Dr. James Lewis*, has observed a not unlike condition of things 

 in the distribution of the fresh-water mussels of Ohio and Alabama. 

 By a series of tables he calls attention to what he believes is the 

 occurrence of identical and equivalent species in the two systems of 

 drainage, and suggests that, owing to the number of varieties charac- 

 terizing the Unio)iidm they may be identical. This author \ has also 

 studied the genus lo and its habits, and notices its variation coinci- 

 dent with latitude and tem2:>erature. 



Dr. R. E. C. StearnSjJ in a paper on the circumpolar distribution 

 of certain fresh-water mussels and the identity of certain species, 

 unites many hitherto recognized species of Anodonta. Dr. J. G. 

 Cooper,* in a study of the fossil and sub-fossil land-shells of the 

 United States, sees the strongest evidence in support of the idea that 

 the older ones are the direct ancestors of certain forms living to-day. 



Mr. R. P. Whitfield || read a paper before the Boston Society of 

 Natural History, showing changes produced in Limnoea megasoma 

 when kept in an aquarium. Having at the outset three specimens, 

 two of them finally died, and from the remaining one eggs were pro- 

 duced, presumably unimpregnated. These eggs hatched, and from 

 these the next year came a second generation, which in turn produced 

 a third generation the following year. The animal of LimiKea is 

 hermaphrodite. Nevertheless, besides diminished size in the shell, it 

 was observed that the male parts had disappeared, and the liver had 

 become considerably reduced in size. He shows that a dioecious 

 species had in a short time become monoecious as a I'esult of the new 

 physical conditions of life in the constricted quarters of an aquarium. 



An instructive paper by D. W. D. Hartman,"^ on the genus Par- 

 tula of the Hawaiian Islands, shows in the most convincing manner 

 the effect of environment in modifying the species. He finds a com- 

 mon occurrence of hybrids among certain forms, the result of the union 

 of proximate species. This hybridization occurring even between 

 arboreal and ground species. Dr. Hartraan states that "gravid females 

 are often washed by heavy rains from a favored position to drier 

 levels, where after a few generations the progeny become depauper- 



* "Proceedings of the Philadelphia Academy of Natural Sciences," 1877, p. 2G. 



f " Aiuerican Naturalist," vol. x, p. 321. 



X " Proceedings of the California Academy of Natural Sciences." 



« Ibid., vol. i, No. 4, p. 235. 



I "American Naturalist," vol. xiv, p. 51. ■*■ Ibid., vol. xvi, p. 581. 



