Such in brief outline is the Darwinian system, which undertakes to account for all the alleged facts of Organic Evolution by means of the above factor, variously described as "Natural Selection," or the "Survival of the fittest in the Struggle for Existence." It should be remembered, though it is constantly forgotten, that it is this particular theory as to the working-cause of evolutionary transformations which is the essence of Darwinism. Mr. Darwin did not originate the idea of genetic transformism, which is almost necessarily suggested by the systematic development of life-forms to which Geology bears witness. Consequently, long before he came on the scene, the doctrine of transformation had been propounded, especially by Lamarck, and if it had met with no general acceptance, this was chiefly because no force was indicated which seemed to offer a satisfactory account of the mode in which the required changes could have been wrought. Such a force Mr. Darwin's "Natural Selection" was widely taken to furnish, and his theory was eagerly welcomed and adopted by those who only required such a basis on which to ground beliefs to which they were already predisposed, and Darwinism thus obtained that pre-eminent position which it still retains, at least in popular estimation. Two special arguments may here be mentioned, which, although they really apply to all systems of Organic Evolution, have obtained a prescriptive right to be quoted particularly in favour of Darwinism, their bearing on which is easily seen.