INORGANIC EVOLUTION. Under this head may be comprised the evolution of the cosmos or mate rial universe; the evolution of our solar system. It is chiefly concerned with the evolution of °sir own planet, in the manner described by the fram ers of the nebular hypothesis. It involves the gradual development of planets from primi tive nebulous masses through the different. gas eous stages of nebulae, which have been hap pily called, by Clodd, "the raw material of which suns and systems are formed." Planetary evolution has to take into account the formation of the air or atmosphere, of water, and the origin of the denser minerals comprised in the mass of planets.
Chemical evolution then follows. This is the gradual evolution, underlaid and conditioned by the physical forces of matter, from elementary or still simpler conditions, through compounds of various degrees of complexity to the most com plex of i. e. protoplasm. In this substance physico-chemical evolution reached its farthest limits. Since life began inorganic chemistry has gone no further (Le Conte). It is now being recognized that something akin to evolution must have taken place in the elements, since the ele mentary atoms postulated by the chemist are themselves supposed to be wonderfully complex aggregates of yet smaller particles.
OanaNic EVOLUTION. As we shall see, this is the theory of descent, or an attempt to account for the origin of organic species. The theory of descent, however—of the origin of species—was the result of attempts after the time of 'Una; to define and classify plants and animals. Owing to the perplexing variations of the living plants and animals, the difficulty of drawing the limits between the more variable species, the multi plication of specimens in our museums, showing a filiation between ninny species, though there were wide gaps between others, it became recog nized by Lamarck that species were artificial. i.e.
ideas; that the individual only was natural or existed in nature, and that the plant and the ani mal kingdoms should be represented by a gene alogical tree, with its stem-forms and later de rivatives.
As the knowledge of species increased, through the sciences of embryology, morphology, paleon tology, and the new light thrown on the varth's history by great advances in geology, a sufficient fullness of knowledge resulted, and almost in a single year, 1859, the' combined researches of the studies of plant and animal life in different quarters of the globe eillminated in the epoch making theory of descent proposed by Darwin, and by Wallace, independently of each other, and seconded by I looker, Fritz Alfiller, Asa Gray, flux ley, and others.
1"tider the head of Organie Evolution we have the factor: or agencies by which variation has been brought about, giving us the materials on which natural selection acts. The great facts in nature are adaptation and variation, and the causes of morphogenesis. of the origin of types and species, and all the actions of the physical agents, such as light, heat, cold; the chemical changes of the medium in which plants and ani mals live; changes in the environment, i.e. cli mate, temperature, altitude, and physiological changes, such as the use and disuse of organs. parasitism, and finally heredity. These are called the 'primary' factors of evolution, while 'natural selection' expresses the results of the action of these primary factors of organic evolution.