ECOLOGY OF ISLE ROYALE. 145 



itant type. This clearly shows that in time diverse local influences have 

 flowed into the general environmental trend or current and have become 

 a, part of it. There is thus a very strong convergent tendency. By 

 convergence is meant the iudependent production of the same kind of 

 association from diverse starting points or habitats and associations. 

 Quite minor ecological units may show similar but temporary con- 

 vergent tendencies in their succession. It is therefore not surprising 

 that any marked environmental dominance will tend to produce simi- 

 lar or convergent results, even in local areas. Under such circumstances 

 similar associationsi or societies may be independently and repeatedly 

 formed by the selecting' environmental influences, such as, for example, 

 are found in the numerous small lakes scattered throughout the coni- 

 fenous forests. This convergent phenomenon is certainly a fertile 

 source of confusion throughout all phases of science. Perhaps the best 

 guide through such a labyrinth will be to clearly bear in mind the rela- 

 tive value of general and local influences, and watch with an "eternal 

 vigilance" for convergent results due to diverse causes. This con- 

 vergent phenomenon is particularly liable to occur in the case of en- 

 vironments produced by reversible physical conditions. It should 

 further be stated that a study of these problems from a genetic and 

 dynamic point of ,view will aid in recognizing such results. Under such 

 circumstances attention is primarily directed toward the dominant 

 causes and conditions of change rather than to the stages, products, 

 and results produced by them. Convergence thus viewed is the result 

 of several causes and should be considered a product rather than a 

 process. This same distinction may be made for all societies, associa- 

 tions and formations. Convergent phenomena are thus partiowlarly 

 liable to confuse wherever products rather than genetic processes receive 

 primary emphasis. 



6. Succession and Environmental Evolution. The relation of suc- 

 cession to general biological problems is very intimate. This opens up 

 a very extensive field which is only mentioned to indicate its general 

 relation to succession. The facts of succession and evolution must ever 

 remain far in .advance of our knowledge of their causes. If, however, 

 one turns to the standard evolutionary treatises and searches for a dis- 

 cussion of the evolution of the environment, as correlated Avith animal 

 evolution, only the most general, or the elementary aaid superficial 

 phases, are as a rule discussed. To be sure, certain papers and treatises 

 take up special phases of the problem, and the broadest phases axe 

 treated by the geologists; but none of them seem adequate as a com- 

 prehensive treatment of so important a subject. Succession, broadly and 

 genetically considered (dynamic rather than static), is a phase of en- 

 vironmental evolution. 



7. The Relation of Succession to Organic Evolution. Mention has 

 been made of the relation of succession to environmental evolution, 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 interpretation. 



One school maintains that all causes of evolution are internal, and 



