INTRODUCTION" 



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 needs, 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. It might be expected 

 that the new form would disappear by crossing with other individuals, 

 but recent work on heredity has minimised this difficulty. A newly 

 acquired character reappears in a portion of the descendants even 

 after crossing, and can thus, if advantageous, be selected. Hugo DE 

 Vries has endeavoured to obtain an insight into the laws of phylo- 

 genetic development by systematic cultivation of particular plants. 

 It would appear from such cultures {*) 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 appear suddenly and are 

 inherited, reappearing according to special laws of heredity in the 

 progeny. 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 {^). 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 resemblances in external form have 

 arisen between organisms living under similar conditions although 

 belonging to different archetypes (''). Natural selection exercises a 

 constant influence on the process and tends to render species distinct 

 by removing the less advantageous valuations. 



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

 sharp distinction between plants and animals 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 composing 

 the plant body, and the green chromatophores within the latter, 

 are important indications of the vegetable character of an organism. 

 Surrounded by firm walls, the living substance becomes more 

 isolated, and, consequently, independence of action in plants, as 



