372 THE POPULAR SCIENCE MONTHLY 



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 environ- 

 ment 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, how- 

 ever, 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 phys- 

 iology we look in vain for an answer to this question, that is perhaps 

 a physiological problem, for while physiology has advanced to a won- 

 derful 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 Dar- 

 winism. 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. 



