philosophy. When presented with Darwin's conception of evolution, the botanist and zoologist could no longer remain satisfied with mere contemplation of privileged moments. It became necessary to attend to every aspect of the organism. Every phase of its development from the egg to its dissolution, and every particle of its structure had to be submitted to the closest examination, for no character—so ran the theory—was too insignificant to decide whether a species could survive in the struggle for existence. Indeed, even vestigial and rudimental characters began to assume an astonishing importance as witnessing to the past and prefiguring the future history of the species. Such close scrutiny of the entire life-cycle and structure of organisms was also necessitated by Darwin's assumption that the evolution of organic forms is a very gradual process, requiring enormous periods of time. Hence the incentive to record the minutest variations and adaptations and to search for their causes. From the same source sprang the inspiration to study the lower animals and plants with the utmost care and in all their aspects, for these forms—according to the theory—had departed least from the first, simple beginnings of life on our planet, and might, therefore, be expected to throw light on the initial movements of organic evolution. But the animals at the other end of the organic scale were not to be neglected, for the theory which made man a blood relation of the higher mammals could not fail to arouse universal interest in these and all the other vertebrates. And not only did it become necessary to investigate the plants and animals now living on the earth, but the strata of the planet had to be ransacked for evidences of past evolutionary history. Paleontology was born anew, and the distribution of life in the present and past became a subject of absorbing and ardent study. Even the most conservative branch of biology, the classification of animals and plants, was shaken to its foundations by the theory of evolution, for under its solvent action the old conceptions of the genus and species, and all the other taxonomic categories, lost their rigid outlines and became fluid and dynamic. In proof of all these statements concerning the immediate effects of the promulgation of the theory of evolution, we have only to glance at the marvelous development during the past fifty years of anatomy, embryology, histology, cytology and physiology, both human and comparative, and of paleontology, chorology, ethology and taxonomy, both botanical and zoological. The constantly increasing tendency during the past half century to substitute a careful genetic study, that is, a study of all the stages of the life-processes, with a view to establishing the laws or constants of their occurrence, for the ancient cut and dried methods of defining and classifying concepts obtained from the contemplation of privileged moments in the vital flux, has spread far beyond the confines of biology properly so-called. Psychology, which is the most closely related of all
