146 MICHIGAN SURVEY, 1908. 



that the environment is only a condition, not a cause. From this point 

 of view the fundamental causes are internal and therefore environmental 

 conditions cam only indirectly influence evolution through the weeding 

 out of those forms not in harmony with tlie conditionsi; and hence it 

 lias a selective rather than an originative influence-. From this point of 

 view succession and environmental evolution can contribute nothing to 

 the elucidation of the causes of organic evolution, though they may to 

 an understanding of the selection produced by the succession of condi- 

 tions in which organic evolution has taken and is taking place. In 

 . harmony with this point of view, succession, broadly treated, should 

 furnish a fundamental method of treatment for the process of selec- 

 tion, and the detailed principles of its working. This would certainly 

 be an important advance because natural selection has frequently been 

 reproached for its indefinite methods and lack of definite ti'eatment. 

 Succession from this point of view is primarily related to the Dar- 

 winian factors of evolution. No doubt this is one reason why Darwin 

 himself put such high value upon the study of ecological relations of 

 animals, i. e., their relation to their complete environment, or their 

 struggle for existence. 



If, however, all causes are internal and not directly subject to ex- 

 ternal influences, they must be beyond experimentation to a. correspond- 

 ing degree. Under such conditions evolution becomes a descriptive 

 rather than a causal science, and all that iuA^estigation can do is to 

 describe the succession of forms produced by these internal causes. 



On the other hand the rival school maintains that both internal and 

 external conditions may be real causes of organic evolution. This is 

 thought to be brought about by the direct or indirect influence of the 

 environiment upon the genn cells, by environmental selection, . or even 

 by both combined. From such a point of view the environment may 

 thus be either a cause or a condition of organic evolution, or both. From 

 such a standpoint the evolution of the environment receives increased 

 importance, as under such conditions organic and environmental evolu- 

 tion are causually related, and thus intimately correlated. Viewed thus, 

 environmental evolution is more than the description of the ' silcoession 

 of conditions, but may be explanatory as well. 



The particularly significant feature is that environmental evolution 

 and biotic succession are of great value and can conti'ibute either to 

 the causes or conditions, or to both, of evolutionary advancement. 



VI. Some Principles of Succession. 



By succession is meant the progxessive change (= adjustment) in 

 the composition of the associations at a given place. If a swamp be- 

 comes filled with dune sand, the birds characteristic of the swamp will 

 be replaced by those of the dunes, and thus succession is initiated. But 

 in addition to changes due to local influences there are those pro- 

 duced by very extensive or geographic influences, as in the case of a cli- 

 matic change. Attention should also be directed to the fact, that biotic 

 succession is only a particular phase of the general law of change 

 which we see operating wherever a complex of forces are tending to- 

 ward a condition of mutual adjustment. That succession is a process 



