ZOOLOGY SINCE DARWIN. 487 



and often extraordinarily complicated assemblage of the remaining 

 organs of the body. It is in this, that in the segmentation that has 

 occurred during the preceding development of the ovum, those cells 

 which are to play the part of germ cells in the future mature organism 

 have assigned to them all portions of the ovum necessary for the recon- 

 struction of the entire body, while to the somatic cells only those sub- 

 stances are assigned which are required for the construction of a 

 definite organ or apparatus. In this way the soma becomes eventually 

 an organ of the propagating cells that completes the work of metabo- 

 lism, moves, feels, and thinks, but yet only serves, as may be said, for 

 the evolution of the germ cells and so assures the continuity of life. 



Since we have seen what are the chief new lines along which zoology 

 has developed since Darwin's time, we may now be permitted to con- 

 sider what modifications have been given to the old problems, once the 

 only ones, namely, the description of the existing material of animal 

 forms and the observation of their life history; that is to say, systematic 

 natural history and biology in the restricted sense. The latter, although 

 it had been for a long time neglected, gave the theory of natural selec- 

 tion a new and mighty impulse. It has been for this science a flourish- 

 ing period, which can only be compared with that epoch at the end of 

 the eighteenth century when were made the striking discoveries of 

 Eeaumur, Roesel, De Geer, Bonnet, Schiiffer, and others. How import- 

 ant now, from a Darwinian point of view, were the correlation of animals 

 with each other and with plants, the influence of climate and food and 

 of light and heat to the struggle for existence, and to the phenomena of 

 natural selection ! The entire world offered material for this study, and 

 there appeared books like H. W. Bates's Naturalist on the River 

 Amazon and A. R. Wallace's Malay Archipelago, which were models for 

 biological studies. A great number of excellent English and German 

 investigators applied themselves to this work and found in the biologic 

 history of plants and animals ever new evidence of the truth of the 

 Darwinian theory, yet at the same time often encountering puzzling 

 phenomena whose elucidation has baffled the acuteness of naturalists 

 even to the present time. It is evident that here, as in other depart- 

 ments, we have not yet reached the end of knowledge, or, better, that 

 of search for knowledge, and of means for investigating the truth. 



Under biological facts there was formerly always placed geographical 

 distribution. This was, indeed, only because it was customary to 

 explain faunal variations by referring their causes to environmental 

 conditions. As these did not usually afford a plausible explanation, 

 zoogeography was essentially a collection of lists whose perusal was of 

 not much more value to zoologists than a glance through a menag- 

 erie. The new foundation for the theory of descent has made a funda- 

 mental chauge in this, as it makes possible for the first time a scientific 

 treatment of the facts of zoogeography. The faunal character of a 

 region is determined by its geological age, the phylogenetic condition 

 of the animal kingdom at the time of its deposition and its varying 



