VARIATION, in biology, the physical de parture in any direction from the mean char acter of a species. When the variation in a large number of individuals, generally more or less isolated in locality, is of a marked and constant type, the group of individuals which exhibits such variation is termed, technically, a °variety?' Variation, strictly speaking, is regarded as distinct from the processes of evolution, which result in the formation of new species, and is limited to the individual variations within the species. Variations may be either collective or individual. The former are those arising from the carrying over of certain peculiarities of in dividual variation from generation to genera tion. They thus tend to become fixed; and re ceive the name of °mutations.° Individual variations may be blastogenetic— that is, in born; or ontogenetic— that is, acquired through the exigiencies of individual environment. In the latter case they are called Discussion has been rife as to whether varia tion is determinate or indeterminate; that is, whether organisms have or have not a tend ency to vary in particular ways. It is the view of the extreme Darwinists that there is primi tively no tendency to any special mode of varia tion, any existing tendency being the result of the selection of those individuals which chanced to vary along these particular lines. Darwin himself held to the idea that adaptation was the secret of such observed changes, and that both the principles of natural selection and of varia tion played an individual part. According to other biological observers and thinkers there is, apart from the guidance of natural selection, an inherent bias, differing in different groups of organisms toward variation in determinate lines. This may be due to the inheritance of characters individually acquired under the stress of surrounding conditions (direct environmental determinism) ; or to constitutional tendencies inherent in the individuals of each species, analogous to the inherent tendencies of inor ganic substances, to assume definite crystalline forms —(innate specific determinism). Accord ing to De Vries there is, in elementary species which cross-breed, a unit characteristic which is not mated; that is, °the differentiating mark is present in one of the parents and not in the other; while all the other units are paired in the hybrid, this one is not. It meets with no mate and must, therefore, remain unpaired. . . These unpaired qualities constitute the essential features of the hybrids of species, and are, at the same time, the cause of their wide deviations from ordinary rules.° It has been claimed by certain American biologists that palmontological evidence estab lishes the existence of determinate variation. The teeth and the limb-bones of more than one series of fossil ungulates are found to exhibit variation along definite and determinate lines.
The facts may •be admitted; but the reasoning based thereon is inconclusive. The variation adduced is confessedly along lines that are ad vantageous to the individuals in which it oc curs. It would, therefore, on the Darwinian theory, escape that elimination which would be the fate of non-advantageous and neutral varia tions. If the teeth of mammals varied inde terminately, and if all variations save those along one line (or several correlated lines) were neutral or non-adaptive, these latter would be eliminated through inter-crossing, while the adaptive variation would become evident. In the fossil forms the variations along non adaptive lines would be so slight as to escape detection, while those in a plus or minus direc ticin along adaptive lines would be assigned to different stages in the evolution of the variation in question.
A. R. Wallace and others have tabulated some results of the observation of variation in the state of nature; and Wallace has shown that variations in size or length of particular parts are considerable, "usually reaching 10 or 20, and sometimes even 25 per cent of the varying and occurring in 5 to 10 per cent of the specimens examined. These results incidentally show that in the species under examination there was no very rigid elimination, and that inter-crossing did not supptess variations from the mean to such an extent as is sometimes supposed.
On the hypothesis of indeterminate variation it must be confessed that we are to-day not much in advance of Charles Darwin, who said: °Our ignorance of the laws of variation is pro found. Not in one case out of a hundred can we pretend to assign any reason why this or that part has varied.° Darwin, however, rigor ously rejected any doctrine of “chance,° insist ing that failure to discover the reasons was due solely to ignorance of well-ordered laws. It is to be remembered that the animal organism is not passive, but reacts individually to environ ment; thus developing a unit characteristic which may be unpaired in mating and so estab lish the foundation for a new variety. Consult the works of Charles Darwin, Lamarck, Weis maim and Wallace (qq.v.) also Bateson, W., (Materials for the Study of Variation' (Lon don 1894) ; Cope, E. D.,
Factors of Organic Evolution' (Chicago 1896) • Daven port, C. B.,
Methods, with Special Reference to Biological Variation> (New York 1904) ; Lloyd, R. E., The Growth of Groups in the Animal Kingdom> (London 1912) ; Lock, R. H., (Recent Progress in the Study of Variation, Heredity and Evolution> (New York 1907) ; Vernon, H. M.,