is dependent on (a) fresh-water communication; on (b) character of stream, that is, of water, as to purity, depth, rapidity, vegetable growth, etc.; on (c) the character of the river-bed, as to size, condition of bottom, etc.; on (d) 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[1] 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 characterizing the Unionidæ they may be identical. This author[2] has also studied the genus Io and its habits, and notices its variation coincident with latitude and temperature.
Dr. R. E. C. Stearns,[3] 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,[4] 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[5] read a paper before the Boston Society of Natural History, showing changes produced in Limnæa megasoma "when kept in an aquarium. Having at the outset three specimens, two of them finally died, and from the remaining one eggs were produced, 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 Limnæa 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 diœcious species had in a short time become monœcious as a result of the new physical conditions of life in the constricted quarters of an aquarium.
An instructive paper by D. W. D. Hartman,[6] on the genus Partula of the Hawaiian Islands, shows in the most convincing manner the effect of environment in modifying the species. He finds a common occurrence of hybrids among certain forms, the result of the union of proximate species. This hybridization occurring even between arboreal and ground species. Dr. Hartman 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-
