CONVERGENCE 309 



themselves. 1 On the land, plants are the dominant sessile forms, and 

 often profoundly modify the conditions in which they live, so that they 

 cannot succeed themselves. When will the process of succession stop ? 

 Obviously, it must cease when there are no available species to take the 

 places of those which have destroyed their own habitats. There are 

 species which are immune to their own products and the products of the 

 species which are associated with them. Obviously, when a condition 

 in which these species can live is reached, and they come to occupy the 

 place which is thus made ready for them, the formation which they 

 constitute can, so far as the plants are concerned, last indefinitely. This 

 is theoretically true of all climax or geographic formations, and has been 

 established for the beech and maple forest of eastern America. 



2. MOTILE AND SESSILE ORGANISMS IN SUCCESSION 

 Motile Organisms Fixed Organisms 



a) Motile organisms affect their own a) Sessile organisms modify their 

 environments by the destruction own environments largely through 

 of materials of abode and food growth of their own bodies, cutting 

 supply and the pollution of their off light, interfering with circula- 

 habitats by waste products (196, tion in surrounding medium and 

 114, and citations). accumulation of waste products 



(195, 120). 



b) The changes under (a) make the ft) The same as for motile organisms 

 continued existence of the group (197). 



in question impossible and pre- 

 pare the way for other differently 

 adapted (succession) forms. 



c) Succession is a succession of c) Breeding and living places are not 

 breeding-places. contrasted as young stages usually 



thrive only where adults can live. 



Succession can take place only where forms adapted to the changed 

 conditions are available. 



3. CONVERGENCE 



The work of running water, for example, is in a measure convergent. 

 When a new body of land is uplifted, streams begin to work their way 

 into the new land mass and cut deep valleys. The formation of numer- 

 ous tributaries (92 and citations) isolates portions of the upland in the 



1 In the sea (195) sessile forms are chiefly animals and animals are probably the 

 chief cause of succession there. Coral polyps cannot build upward indefinitely, as 

 they soon reach the surface and can no longer exist. By reaching the surface they 

 prepare the way for other forms. 



