CONDITIONS OF THE STRUGGLE 349 



which forms the basis of one part of his hypothesis concerning the origin 

 of species. 1 Yet other writers had previously noted this struggle in 

 nature ; for instance, A. P. de Candolle wrote : ' Toutes les plantes d'un 

 pays, toutes celles d'un lieu donne, sont dans un etat de guerre les unes 

 relativement aux autres.' 2 



The struggle and competition among plants are brought into greater 

 prominence by changes that continue without interruption in soil, climate, 

 or other conditions affecting plant-life, including changes in the animal- 

 world. 



Without such changes the results of the struggle would be neither 

 so distinct nor so rapid. The changes in question are 



i. Production of new soil. 



ii. Changes in old soil, or in the vegetation covering it, and in the 

 factors discussed in the first Section of this work, but particularly those 

 caused by man, who is thus responsible for ' semi-cultivated ' formations. 

 Man intervenes directly when he utilizes the soil for his own purposes, 

 by converting forests into arable ground, or by draining moors, but he 

 also intervenes indirectly when permitting cattle to graze, or when he 

 mows, manures the soil, and so forth. 



In regard to the question now under discussion reference should be 

 made to Clement's interesting work, ' The Development and Structure 

 of Vegetation ' (1904). In discussing the migrations and invasions of 

 plants Clements distinguishes between migration and ' ecesis '. ' Migra- 

 tion merely carries the spore, seed, or propagule into the area to be in- 

 vaded ; ecesis is the adjustment of a plant to a new habitat, it is the 

 decisive factor in invasion, inasmuch as migration is entirely ineffective 

 without it.' In discussing invasion he treats of barriers, endemism, poly- 

 genesis, also manner and kinds of invasion. 3 



The struggles between communities and the development of these 

 are elucidated by means of examples in the succeeding chapter. 



CHAPTER XCV. THE PEOPLING OF NEW SOIL 



WHEN new soil arises anywhere it is soon invaded by plants. And it 

 is of deep interest to follow the successive phases in the development of 

 the vegetation. In this way one will acquire evidence of a long series 

 of struggles among the successive immigrants ; these struggles sometimes 

 do not end in any decisive result before the lapse of many decades. 



New soil arises in the following places : on coasts, where the sea 

 deposits fresh material ; at the mouths of rivers ; even in river-beds, 

 where masses that have been washed down are deposited. New soil also 

 arises by the following agencies : action of glaciers, talus, volcanic erup- 

 tions, fire that devastates the original vegetation, and human action, but 



' See also J. D. Hooker. 2 A. P. de Candolle, 1820. 



3 See also MacMillan, 1893, 1896, 1897 ; Shantz, 1906, 1907 ; Cowles, 1899, 

 1901 ; Clements, 1905 ; Adams, 1905 ; A. Nilsson, 1899 ; Whitford, 1901. The 

 means of migration of plants are dealt with in papers by Guppy, 1003, 1906; 

 Hemsley, 1885 ; Jouan, 1865 ; Schimper, 1891 ; Hildebrand, 1873 ; Sernander, 

 1901; Vogler, 19016; J. Holmboe, 1898; Ravn, 1894 ; and many others. 



