INTRODUCTION 



due to natural causes that adaptability to the environment which is 

 such a striking characteristic of organic life. That the transitional 

 forms in this process of phylogenetic development no longer exist, is 

 accounted for in the theory of natural selection by the assumption 

 that the struggle for existence must necessarily have been most 

 severe between similar organisms. For similar organisms have 

 similar necessities, and the new and better-equipped forms must 

 ultimately prevail over the original less specialised organisms and 

 exterminate them. 



Since the publication of Darwin's works many investigators have 

 laboured to advance and make clear our views on phylogeny. 

 Difficulties in applying the results of artificial selection to the natural 

 process became evident, for one main condition of successful artificial 

 selection, the isolation of the organisms from which breeding is 

 taking place, is not fulfilled under natural conditions. Of late years 

 HUGO DE VRIES has endeavoured to obtain an insight into the laws 

 of phylogenetic development by systematic cultivation of particular 

 plants. It would appear from such cultures ( 4 ) that the starting- 

 point for the origin of new species is not afforded by the " fluctuating 

 variations," which continually occur, but by more marked variations 

 which have been termed " mutations " ; these mutations appeal- 

 suddenly and are strongly inherited. On the other hand it may be 

 said that a sharp line cannot be drawn between mutations and 

 fluctuating variations. DE VRIES tended to assume the existence of 

 a development of the organic world due to original innate capabilities 

 of the living substance, and not dependent on selection. The origin 

 of the large subdivisions of the animal and vegetable kingdoms, the 

 "archetypes," would be due to this sort of evolution ( 5 ). The 

 organisms have been, and are still, continually influenced by the 

 environment, and by their reaction to external conditions have 

 become more or less directly adapted. In this way striking re- 

 semblances in external form have arisen between organisms living 

 under similar conditions although belonging to different archetypes ( 6 ). 

 Natural selection exercises a constant influence on the process and 

 tends to render species distinct by removing the less advantageous 

 variations. 



If the higher organisms have been evolved from the lower, a 

 sharp distinction between plants and animais is excluded. For the 

 characters which are distinctive of animals and plants have appeared 

 in the course of the phylogenetic development of organisms, and 

 were at first wanting. The simplest organisms which now exist are 

 in all probability similar to those which formed the starting-point 

 of this development. The walls which surround the cells com- 

 posing the plant body, and the green chroma tophores within the 

 latter, have been cited as decisive indications of the vegetable 

 character of an organism. Surrounded by firm walls, the living 



