The assumption of Darwin and Wallace that variations are haphazard in origin and indefinite in direction—If these assumptions be not correct Natural Selection ceases to be the fundamental factor in evolution—Darwin’s views regarding variation underwent modification—He eventually recognised the distinction between definite and indefinite variations, and between continuous and discontinuous variations—Darwin attached but little importance to either definite or discontinuous variations—Darwin’s views on the causes of variations—Criticism of Darwin’s views—Variations appear to occur along certain definite lines—There seems to be a limit to the extent to which fluctuating variations can be accumulated—De Vries’ experiments—Bateson on “discontinuous variation”—Views held by De Vries—Distinction between continuous and discontinuous variations—The work of De Vries—Advantages enjoyed by the botanist in experimenting on the making of species—Difficulties encountered by the animal breeder—Mutations among animals—The distinction between germinal and somatic variations—The latter, though not transmitted to offspring, are often of considerable value to their possessor in the struggle for existence. Nature of Variation As we have already seen, the Darwinian theory, unlike that of Lamarck, does not attempt to explain the origin of variations. It is content with the fact that variations do occur. Although Darwin did not try to explain how it is that variation occurs, and was very guarded in the expressions he used concerning it, he assumed that variations are indefinite in variety and occur indiscriminately in all directions, as the following quotations from the _Origin of Species_ will show: “But the number and diversity of inheritable deviations of structure . . . are endless” (page 14, ed. 1902). “The variations are supposed to be extremely slight, but of the most diversified nature.” “I have hitherto sometimes spoken as if the variations so common and multiform with organic beings under domestication, and in a lesser degree to those under nature, were due to chance. This, of course, is a wholly incorrect expression, but it serves to acknowledge plainly our ignorance of the cause of each particular variation” (page 164). Wallace is far less guarded in his expressions. On page 82 of his _Darwinism_ he speaks of “the constant and large amount of variation of every part in all directions . . . which must afford an ample supply of favourable variations whenever required.”