CONVERGENCE 309 
themselves.t 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 6) 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 
tIn 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. 
