THE DIFFERENTIATION OF GEOGRAPHICAL GROUPS IN LYMNAEA PALUSTRIS

¹ From the Department of Zoölogy, Barnard College, Columbia University

Variation in Lymnaea palustris is exhibited in fertility, rate of growth, size, and longevity. Individual, family and group differences recur in successive generations in spite of the essential uniformity of the laboratory conditions of culture; hence their genetic causation seems probable.

Sample populations collected from two different localities and habitats were analyzed by the F test for significant differences in fertility,

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growth, and longevity. The results demonstrated the reality of definite differences in all three categories.

The study of group differences was continued throughout three consecutive offspring generations, and in each of these the differences demonstrated in the parent generation were found to be repeated. The two local populations are therefore to be regarded as different geographical races.

Significant and positive correlations were found between the total number of eggs laid and the age at death, between the number of eggs laid and the length of the fertile period, and between the length of the fertile period and age at death.

Extreme types of slow-growing and fast-growing individuals were found in every group with intermediates of great variety. When arranged in order of size at 80 days, the snails exhibited a tendency to keep their respective places throughout the rest of the life cycle, as indicated by the positive correlation between size at 80 days and size at 320 days. The rate of growth between 80 days and 160 days is inversely correlated with size at 80 days. The critical period in the growth curve occurs relatively early in the life cycle when the individual mode of growth appears to be established.

The onset of fertility is negatively correlated as to time with size at 80 days. The correlation in question is the same in the two local series, despite their demonstrated differences in size at 80 days and in age at the onset of fertility.

Some of the individuals in each population were sterile. When a study was made to ascertain whether such animals differed significantly from the fertile snails, the F test showed that they were collectively smaller than the productive individuals and that their lives were shorter.