ECOLOGICAL SUCCESSION. 353 



physical conditions. A reviewer (Pearse, '12) of Hancock's 

 book (Shelford, '12) appears to have thought that this classi- 

 fication followed that of plant ecologists, but the main difficulty 

 with this classification and that of Morse, is that they do 

 not follow the plant ecologists' classification in any of its 

 essential features. Both made the primary division vertical, 

 though frequent invasion of several strata by the same animals 

 makes the application of such a system to entire communities 

 a practical impossibility. Such invasions of the recognizable 

 horizontal divisions are far less frequent. Dahl ('08), in a 

 list of places from which animals may be collected, gives a 

 good classification of animal habitats and partially separates the 

 primary and the secondary conditions. His general outline 

 possesses many points of merit, and is probably the best 

 published list. The author's comprehensive knowledge of 

 the subject is clearly indicated throughout. It is not, how- 

 ever, arranged with particular reference to physical factors, 

 and differs from the attempts of the American ecologists in that 

 it is not based upon the laws of evolution or succession of 

 environments. 



The advantages of following natural laws, in ecological classi- 

 fication, should not need elaboration, for such has been the chief 

 guide of all systems of classification of scientific data, at least since 

 the time of Darwin. Elements of progress in ecological classi- 

 fication, following natural laws governing animal and plant 

 habitats, have come chiefly from American ecologists, and largely, 

 no doubt, because in the newer country there has been more 

 opportunity to study the laws of succession. In Europe such 

 laws can more rarely be studied, because of the more intensive 

 efforts of man to prevent the operation of such laws. 



In this country, Cowles was the first to make use of the laws 

 governing plant habitats, in the classification of plant communi- 

 ties. Warming who contributed to the field before Cowles, took 

 the condition of the plant as his main guide, but clearly recognized 

 the laws governing habitats as of importance. Clements has also 

 added much on the plant side. 



Adams ('08) and Gleason ('08) arranged the animal communi- 

 ties of Isle Royale, Lake Superior, according to laws of succession, 



