Summary.
Evolutionary study and thought have been hindered by the confusion of two unrelated biological phenomena, (1) evolutionary progress or vital motion, and (3) the origination or multiplication of species. The 'origin' of a species is not more evolutionary than any other stage in its history. The causes of the subdivision of species are not causes of vital motion; the two processes are quite distinct. The separation of two species is not a focus of the evolutionary problem; it is a mere incident of developmental history.
Segregation is the principle or active cause of the multiplication of species, but the nature and causes of evolutionary progress are not to be ascertained by discovering that species originate by subdivision. Vital motion is continuous, and is neither actuated nor interrupted by the segregation which multiplies species.
Natural selection may assist in the segregation of species, but it is not a factor in evolutionary progress, except as it influences the direction of vital motion. Specific groups become diverse when the component individuals no longer share their variations through interbreeding; not because new characters are induced by external influences. Evolutionary divergence may take place under identical conditions, and in characters which have no relation to the environment and no value to the organism except to permit the necessary vital motion.
A stationary heredity or the continued repetition of an identical structural type exists nowhere in nature; variation is an inherent evolutionary property. Segregation is not necessary for the preservation of variations; genetic variations are prepotent and are more rapidly propagated by crossing with the parent form.
A second evolutionary property of organisms is symbasis, which has built up the complex structure of the higher animals and plants by combining individuals into the interbreeding groups called species. The evolutionary species is not a complex of characters or a mere aggregation of similar plants or animals; it is a protoplasmic network held together by the interbreeding of the component individuals. Symbasis accelerates vital motion, but hinders the multiplication of species.
Species and evolution are different aspects of the same fact; evolution goes forward within specific lines as a manifestation of the same property which necessitates the existence of species; variation and cross-fertilization are not antagonistic phenomena, but two phases of the same creative process.
