branches of zoology we find activity going on in many lines of work. One group of workers, the systematists, have kept nearer, I think, to the older traditions. They have been concerned with three of the most important matters that have a direct influence on the "Origin of Species "—the intensive study of species and varieties, the geographical and geological distribution of animals, and the influence of the environment in modifying species. Their results have supplied the most extensive contributions, perhaps, that have been made to the theory of species-formation and transmutation. They seem to me, however, to have paid less attention to another, equally important, field, that of the adaptation of animals to their environment, and the causes that have been effective in bringing about this adaptation. To physiology we look in vain for an answer to this question, that is perhaps a physiological problem, for while physiology has advanced to a wonderful degree our knowledge of the complicated adjustments within the body, the origin in time of these adjustments and their relation to the outer world has excited less interest.
The morphologists, or philosophical anatomists, form the second great group of students whose activity is a direct outgrowth of Darwinism. The determination of the relationships of the great classes of animals on the principle of descent has occupied much of their time. Two other important fields of labor have also fallen to their share. The study of development or embryology has been almost exclusively pursued by morphologists, inspired in large part by the theory of recapitulation.
The older form of the doctrine, that in the development of the individual the past history of the race is repeated, has been revived—a doctrine much in vogue in the early part of the last century, which has continued to have its followers despite the different interpretation that von Baer gave to the same facts. Whatever interpretation we choose at the present time, the presence of structures like gill-slits in the human embryo, directly comparable to those in the fish, has had an important influence in disentangling the relationship of living animals to their remote ancestors.
The morphologist has also undertaken the study of heredity, and the relation of heredity to the germ-cells that are the links in the chain of organic life. Few other studies have advanced in recent years at a more rapid pace and few have yielded facts of greater significance, for here lies the key to the origin and nature of variations.
Systematists and morphologists alike have been evolutionists, but it is a curious fact of zoological history that until very recently there has been no body of students whose interests have been directed primarily towards the problems of evolution. This is due, I think, to a general feeling that the data for evolution are rather the by-products of the zoologist's work-shop, than products directly manufactured by him.