Ordinary variation is a fact that can not be disputed. No two plants or animals are exactly alike. The amphimixis or the blending of the inheritances of two individuals is, according to Weismann, the great factor in the production of variations. The two parents of every animal or plant have the species-character in common, but there are certain distinctive traits that hybridize. Hence ordinary variation is a fact, and Nature by selection may evolve, in a slow way, new species, just as Luther Burbank creates new kinds by artificial selection. Favorable variations are then bound to furnish the possessor with a greater power of resistance and with higher possibilities of life and propagation. Evolution, accordingly, occurs primarily through sudden mutations or sports which are the fittest for survival. While the principle upon which the Lamarckian doctrines rest is the power of adaptation, the basis of evolution for the Darwinian is the transmissibility of unlikeness or individuality just as likeness. Acquired characters are not transmitted; each generation has to make a fresh start. It does not begin where the last generation has left off. But variations are transmitted to the offspring, and evolution proceeds by sports or by the transilient variation. A serious objection to this theory is the tendency of Nature to revert to the normal average of the race. The law of Galton means the return to the mean of the species. The children of the sport tend to return towards the mean of the race. Thus the four explanations do not satisfactorily explain, and the subject of the method of evolution is not yet decided. Neither teleology with the initiatory psychic energy working towards definite ends, nor Bergson’s vital impulse or original profound cosmic force, nor Lamarck’s appetency or use and disuse, nor Darwin’s natural selection, furnish an unobjectionable satisfactory explanation of organic evolution. Still the world’s thinkers and scientists have accepted organic evolution as a fact which may be proven by embryonic development, in conformity with Haeckel’s biogenetic axiom that ontogeny is only a short recapitulation of phylogeny. The ovum or even the zygote (i. e., the impregnated ovum) is a single-celled organism and resembles the animals of the first or lowest type in the animal kingdom. The protozoa are nothing else than single-celled animals. Some of them have even a lower structure than the common cell. The Monera, e. g., has neither nucleus nor membrane. The manifestations of life are recognizable only by the possession of the faculties of the assimilation of food and of propagation by segmentation and division. [Illustration: CUT I. a, cell; b, morula; c, blastula; d, gastrula; ec, ectoblast; en, endoblast.]