VARIATION IN NATURE The variation of animals and plants in nature is, so far as known, subject to the fundamental laws revealed by experiment and pedigree-breedings (see HEREDITY). Only by such experiments and culture is it possible to decide authoritatively how a given variation is produced and whether it is heritable or not. Never theless it is desirable to know—(a) the actual kinds of variation that occur in nature, (b) which of these we may assume to be heritable and (c) the effect which isolation (topographical, habi tudinal, etc.) and interbreeding may have on the distribution of variant characters in nature. Variation in the habits of animals, the food they eat, their breeding-seasons and all their complex inter-relationships is moreover of cardinal importance in evolu tionary studies and can only be effectively observed among animals in nature.
The kinds of structural variation that occur in nature will be best illustrated by means of the following classification which, although it is not exhaustive, will show the circumstances in which such variation typically occurs.
I. Variation regularly occurring in the presence of a given environmental condition or factor.
(a) Gross variation in form and size associated with the excess or deficiency of some tissue-forming substance.
The periwinkle, clam and mussel when living in water of reduced salinity (e.g., in the Baltic Sea) are found to be smaller than in normal sea-water. Low night-temperature and inferior soil render various hawkweeds (Hieracium) living at high altitudes more stunted than those living at lower altitudes.
(b) Special modifications.
The tail-lamellae of the brine shrimp (Artemis saline) are reduced in water of high salinity. The shape of the common cockle (Cardium edule) is more elongate in water of salinity above that of normal sea-water. Terrestrial animals living in damp situations or in humid atmosphere are often darker than repre sentatives of the same species which live in drier conditions.
This is recorded among slugs (Leydig), birds (Swarth) and mammals (Sumner). The greater part of the variation exempli fied by (a) and (b) is in all probability non-heritable. Experi ment has however shown that in a few cases variation induced by external causes may be inherited either permanently (Lepi doptera, Harrison and Garrett) or tempo rarily (Amphibia, Kammerer; Crustacea, Agar). Similarly the colour-differences recorded by Sumner (see above) have been shown to be hereditary.
A series of allied varieties or races which replace each other in succession, either in time or place, will sometimes exhibit pro gressive modification. Such variation may be simply correlated with a change in geo graphical position, the causes of structural change being obscure.
2. Variation resulting from adaptation to diverse conditions.
3. Variation known to be heritable but not referable to known external causes.
Many variations in colour, form and pattern that are known by experiment to be hereditarily stable have been recognized in nature. Such variations may be distributed at random through a large natural population, or the latter may be divided into topo graphically isolated races or colonies, each of which has a more or less distinct appearance. These divergences may likewise be associated with other modes of isolation, e.g., with mutual sterility, difference in breeding-period, etc. Other examples of this kind of variation are to be found in the "colour-phases" of mammals (e.g., foxes) and birds. Stresemann's important work on the occurrence and heredity of such colour-phases in the latter should be consulted (Bibliography). It is also known in flowers.
4. Variation due to accident, interference with growth processes, etc.