It is not always easy to differentiate between this type of variation and that described in (r). For example the shells of the water snail Planorbis seem to assume the uncoiled ("scalari form") form in water of un usually high temperature. This may be described as an acci dental malformation, but it is ac tually produced by a particular state of the medium in which these snails live. Scalariformity is more obviously "accidental" when produced in the Gastropod Troches by parasitic infection. Plant lice sometimes produce green colour in flowers and the attacks of a rust fungus (Aecid ium) produce shorter and broader leaves in Euphorbia cyparissias.
5. Seasonal Variation.
This is a familiar phenomenon among vertebrate animals, and is also known among invertebrates (Insects, Cephalopods). The change of coat-colour in the mountain hare from "smoky-brown" in summer to white or "blue" in winter is a notable example. In the rufous horseshoe bat of India (Rhinolophus rouxii) the fur after the spring and autumn moults is mouse-brown on the upper surfaces. This changes in course of time into brighter colours, probably owing to oxidation of the pigment.
• 6. Variation due to sexual dimorphism and growth-changes.
Strictly speaking this should not come under the rubric of variation as described here; but it is important to notice that some of the differences in size, colour and shape among animals of the same species may be due to these causes.
The data presented above relate only to the structure of animals and plants. Although the distinction between structural characters and those of physiological constitution and habits is arbitrary, it is convenient to retain the distinction in a survey of this kind. Our knowledge concerning modes of variation other than that of structure is less complete. Each species is probably
as variable in its physiological constitution as it is in its structure. This has been shown by exact tests, and also may be inferred from such facts as the infection of hosts belonging to different families and genera by individuals of the same species of parasite, the occupation by members of the same species of habitats radically different in physical and bionomic factors, varying susceptibility to disease and so on. The variability of habits may be illustrated by Darwin's observations on the behaviour of the great titmouse, certain individuals of which species sometimes behave as birds of prey.
A number of highly important considerations are omitted from this discussion. The problem of intermediacy, the degree of variability in different species of the same genus, the relation between natural variation and classificatory units, observations on the effect of Natural Selection and on the first appearance, increase or decrease of variant individuals are dealt with in the undermentioned works.
BIBLIOGRAPHY (Select).—W. Bateson, Problems of Genetics, 1913 ; Bibliography (Select).—W. Bateson, Problems of Genetics, 1913 ; C. Darwin (various works, passim) ; G. Turesson, Papers on variation in Plants, Hereditas Bd. III.—VIII. (1922-26) ; G. C. Robson, The Species Problem, 1928 ; E. Stresemann, 1926, Journal fur ornithologie LXXIV., p. 3 ff. See also special papers in journals devoted to Genetics and Variation ; P. Pelseneer, "Les variations . . . chez les Mollusques." Mem. (8°) Ac. Roy. Bruxelles, 2, V. 1920, is a valuable collection of data on the variation of a single phylum of animals.
(G. C. R.)