146 Adams, Ecological Succession of Birds. ['\i''^ii 



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 environment upon the germ cells, by environmental selection, 

 or even by both combined. From such a point of view the environ- 

 ment may thus be either a cause or a condiiion of organic evolution, 

 or both. From such a standpoint the evolution of the environment 

 receives increased importance, as under such conditions organic 

 and environmental evolution are causally related, and thus inti- 

 mately correlated. Viewed thus, environmental evolution is more 

 than the description of the succession of conditions, but may be 

 explanatory as well. 



The particularly significant feature is that environmental evolu- 

 tion and biotic succession are of great value and can contribute 

 either to the causes or conditions, or to both, of evolutionary ad- 

 vancement. 



VI. Some Advantages of a Knowledge of the Lam's of 



Succession. 



The study of succession implies a detailed knowledge of the field 

 relations of birds, and as this has received so little attention as a 

 subject of special study, it is perhaps worth while to briefly mention 

 some of the practical and scientific advantages which we may reason- 

 ably expect will result from the development of this phase of investi- 

 gation. 



The current discussions of environments are generally very frag- 

 mentary and chaotic, and the careful study of bird habitats and 

 succession will greatly improve this phase of ecology. Here is a 

 field of study in need of distinct recognition as a subject worthy of 

 detailed investigation, in addition to those lines already current. 

 ^Yhen once this field is developed, then and only then will it be 

 possible to intelligently discuss the evolution of avian environments 

 and to correlate them with the evolution of birds themselves. It is 

 quite probable that one of the main conditions which prevents a 



