Fossil Records of Mendelian Mutants

Abstract

WITH the help of Prof. A. E. Boycott and others, I have been engaged for the past ten years in collecting data for the study of the distribution of variations in natural populations. The species selected for study were the two common and very variable British landsnails, Helix (Cepæa) nemoralis L. and H. (Cepæa) hortensis, Müll. These snails normally occur in populous and well-defined colonies. In habit they are sedentary and apparently seldom move far from their birthplace. Most of the variations taken into consideration are those that affect the colour and banding of the shell. The classical experiments of Lang, and the wider, but as yet mainly unpublished, work of Mr. A. W. Stelfox, have shown that several of the colour and band forms are inherited on simple Mendelian lines. Both species are recorded fossil from the Red Crag of Butley (Pliocene) and subsequent strata, and C. nemoralis from the Miocene of France (A. E. Ellis, “British Snails”, 1926). Mr. J. W. Taylor in his monograph of the land and freshwater Mollusca of the British Isles gives numerous records of the geological distribution, and says of C. hortensis that in a pleistocene deposit (Ightham, Kent) the five-banded and unbanded forms were about equally numerous. To-day, bandedness is known to be a simple recessive to unbandedness in both species. Mr. W. J. Wintle (Proc. Malac. Soc., 16, 171-8; 1925) reports that on Caldey Island, C. nemoralis is very common as a living snail and as a pleistocene and holocene fossil. He states that among the living snails the unbanded form seems rather more common than the banded forms but that the reverse is true of the pleistocene shells. In neither of these cases, however, are any figures quoted.