To live, then, is to be adapted for successful warfare; and the question as to the mode of origin of species may be restated as an inquiry into the origin of the manifold adaptations by which species are enabled to meet the conditions of life. Why is adaptation a universal phenomenon of organic nature? The answer to this query given by Darwinism may be stated so simply as to seem almost an absurdity. It is, that if there ever were any unadapted organisms, they have disappeared, leaving the world to their more efficient kin. Natural selection proves to be a continuous process of trial and error on a gigantic scale, for all of living nature is involved. Its elements are clear and real; indeed, they are so obvious when our attention is called to them that we wonder why their effects were not understood ages ago. These elements are (1) the universal occurrence of variation, (2) an excessive natural rate of multiplication, (3) the struggle for existence entailed by the foregoing, (4) the consequent elimination of the unfit and the survival of only those that are satisfactorily adapted, and (5) the inheritance of the congenital variations that make for success in the struggle for existence. It is true that these elements are by no means the ultimate causes of evolution, but their complexity does not lessen their validity and efficiency as the immediate factors of the process. * * * * * Taking up the first proposition, we return to the subject of variation that has been discussed previously for the purpose of demonstrating its reality. The observations of every day are enough to convince us that no two living things are ever exactly alike in all respects. The reason is that the many details of organic structure are themselves variable, so that an entire organism cannot be similar to another either in material or in functional regards, while furthermore it would be impossible for an animal to be related to environmental circumstances in the same way as another member of its species unless it was possible for two things to occupy the same space at the same time! Individual differences in physical constitution are displayed by any litter of kittens, with identical parents; it needs only a careful examination to find the variations in the shape of the heads, the length of their tails, and in every other character. Sometimes the differences are less evident in physical qualities than in disposition and mental make-up, for such variations can be found among related kittens just as surely as among the children belonging to a single human family.