^°1908^^] Adams, Ecological Succession of Birds. 145 



the problem, and the broadest phases are treated by the geologists; 

 but none of them seem adequate as a comprehensive treatment of so 

 important a subject. Succession, broadly and genetically consid- 

 ered (d;yTiamic rather than static), is a phase of environmental evolu- 

 tion. 



7. The Relation of Successio7i to Organic Evolution. IVIention 

 has been made of the relation of succession to environmental evolu- 

 tion, but its relation to the organic evolution of birds should also be 

 indicated. The mutual relations of organic and environmental 

 evolution have been and will continue to be the battleground of 

 biological thought for an indefinite length of time. Here lies the 

 tension line between the two main schools of biological interpreta- 

 tion. 



One school maintains that all causes of evolution are internal, 

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

 this point of view the fundamental causes are internal and there- 

 fore environmental conditions can only indirectly influence evolu- 

 tion through the weeding out of those forms not in harmony Avith 

 the conditions; and hence it has a selective rather than an origi- 

 native influence. From this ])oint of view succession and environ- 

 mental 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 conditions in which 

 organic evolution has taken and is taking place. In harmony Avith 

 this point of vitw, succession, broadly treated, should furnish a 

 fundamental method of treatment for the process of selection, 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 treat- 

 ment. Succession from this point of view is primarily related to the 

 Darwinian factors of evolution. No doubt this is one reason why 

 Danvin himself put such high value vipon the study of ecological 

 relations of animals, i. e., their relation to their complete environ- 

 ment, or their struggle for existence. 



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

 external influences, they must be beyond experimentation to a 

 corresponding degree. Under such conditions evolution becomes, 

 a descriptive rather than a causal science, and all that investigation 



