THE variability of animal bodies is a very evident fact. The individuals of every species show variety in color, form and size. Three types of variability have been discovered; fluctuating variation obeying the laws of chance, mutation appearing as sudden loss or gain of a color or other feature, and acquired characters gained by an individual in relation to its surroundings. Among these three types are sought the great factors of evolution. It is a singular fact that no great biologist has attempted to use all three of these factors as the basis of his system, but each author has sought to build his hypothesis upon some one all-important factor.
Fluctuating variation is undoubtedly the greatest of these factors in the part it has played in the history of evolution. It was made by Darwin the corner-stone of his theory when he claimed that natural and artificial selection could produce almost unlimited effects by the elimination of all but the most favorable among thousands of variants in a species. In the debates over the general theory of evolution there has been no argument more often used than the plausibility of Darwin's theory of the survival of the fittest. The public, in accepting the truth of the theory of descent, has come to look upon this factor of fluctuating variation as a necessary part of evolution. In fact, to many professional biologists Darwinism has become synonymous with the survival of the fittest variations.
The theory of mutation is the most serious opponent of the Darwinian theory of selection of variations. Based at first on the evidence gathered by De Vries, it has grown in popularity with the growth of the knowledge of the inheritance of unit characters, and with the discovery of pure line inheritance. In the minds of many biologists it has the advantage of showing a method of rapid evolution more or less independent of the guidance of natural selection. The more ardent supporters of the theory have claimed for it the position formerly held by the theory of fluctuating variations, trying to show that all evolution must be in the nature of loss or gain of unit characters.That the familiar acquired characters of animals should be inherited was once taken for granted, and, in fact, is still a general belief in the world at large. This theory was held by Lamarck to be a great law of evolution. It was defended by Spencer, and assumed occasionally even