308 ECOLOGY 
expect lack of accord within and between plant and animal communities 
under such conditions. In these cases, however, conditions are only 
temporarily out of adjustment, due to rapid physiographic changes, and 
we note from the data presented that plant and animal communities 
are usually in agreement. The exceptions are often apparent only and 
due to the emphasis of species instead of mores and growth form. From 
this viewpoint and with such exceptions as are noted, plant and animal 
communities are probably in agreement the world over. 
IV. RELATIONS OF COMMUNITIES 
I. SUCCESSION—CAUSES 
Succession is no doubt one of the most important and widespread 
of the phenomena discovered by the ecologists up to the present time 
(120,197). Simply stated, it means that on a given fixed area organisms 
succeed one another, because of changes in conditions. These changes 
make impossible the continued existence of the forms present at any 
given time; with the death or migration of such forms, others adapted to 
the changed conditions occupy the area, whenever such adapted forms 
are available. The changes referred to result from physical or bio- 
logical causes, or combinations of the two. It is probable that the causes 
of the changes are frequently complex combinations of various factors. 
We have among the physical causes changes in climate and changes 
in topography. All degradation of land is a cause of succession. Such 
geological processes are well understood and treated in textbooks on 
geology and physiography. 
The biological causes of succession lie chiefly in the fact that organ- 
isms frequently so affect their environments that neither they themselves 
nor their offspring can continue to live at the point where they are now 
living. Every organism adds certain poisonous substances to its sur- 
roundings, and takes away certain substances needed by itself. It 
frequently thus so changes conditions that its offspring cannot live and 
grow to maturity in the same locality as the parents. However, by 
these same processes it prepares the way for other organisms which can 
live and grow in the conditions thus produced. 
Obviously, those organisms whose decaying bodies and excretory 
materials are not removed or distributed by their wanderings will 
modify their environments most. Organisms which remain in one 
place do nothing which tends to remove the results of their own existence, 
and frequently modify their environments in manners detrimental to 
