The Variation of Two Species of Lymnaea.

The problem of the effects of isolation in relation to the establishment of subspecies and local races has been studied by many workers since the time of DARWIN and WALLACE, but in most instances it has been attacked only from the standpoint of geographical speciation. Little attention has been paid to the relationship existing between the type of variation in any one circumscribed locality, and that occurring over extensive geographical areas. The naturalists of sixty years ago were interested in the gross effects of isolation, as exemplified, for instance, in the fauna of oceanic islands, but paid no great attention to the possibility of studying this phenomenon over broad and continuous areas. The work presented in this paper covers an extensive territory in the Sub-Arctic region, and in certain respects runs parallel to the extensive researches of CRAMPTON (1916) upon the members of the genus Partula in certain Pacific islands, but with the important difference that CRAMPTON'S work covers a very small subtropical area. It appears that the differentiation of new species and races takes place in the first-mentioned region a t a much slower rate than in that with the warmer climate, and this renders it possible, in certain respects, to follow the process with more precision. The analysis of this phenomenon in the northern part of North America may therefore be of interest. The ideal animal with which to commence a study of this kind is one which has a very wide geographical range, a considerable tolerance of diverse environmental conditions, and a marked tendency toward variation. For comparison with this it is desirable to use a species which, while equally variable in form, is more specific in its habitat requirements. These conditions are admirably fulfilled by two species of fresh water mollusca, namely, Lymnaea palustris (MULLER), and L. emarginata Say. L. palustris is a hardy and widely distributed snail which is found in North America, Europe, and northern Asia; while L. emarginata is much more restricted in its range, being found only in the northern part of North America east of the Rocky Mountains. A comparison of the local and geographical variation of these two species shows that while in L. palustris the range of variation in one locality approximates that which occurs over almost the whole range of the species in North America, in L. emarginata there is a well marked tendency for the range of variation in any one locality to constitute only a small part of the range which is to be found over the whole