128 MICHIGAN SURVEY, 1908. 



"zones" to the general problem of succession. Such "zones" are thus 

 only particular phases of succession. 



It is thus seen from the above outline of literature that habitat pre- 

 ferences have been outlined for a few widely separated localities and 

 for some agricultural conditions, but there has been no comprehensive 

 , discussion of the problems of habitats and succession, even in a pre- 

 liminary manner, either from) a scientific or economic standpoint. This 

 fact seems rather remarkable in view of the great utility of a knowledge 

 of the general principles underlying economic practice. There are, 

 however, certain phases of biotic succession which have been discussed 

 by a few authors. These subjects have either been discussed in a very 

 general, manner or ai*e detailed discussions of special regions or groups 

 of plants and animals. For this reason, perhaps, their bearing upon 

 other groups than those specifially mentioned are very likely to be over- 

 looked by those who take little interest in any subject or discussion 

 which does not specifically mention their specialty or locality. This 

 phase is mientioned in order to show that while avain successions have 

 been considerably neglected, advances have been made elsewhere, by 

 means of which some general principles appear to have been fairly 

 well established. This is particularly true of plant succession, as 

 shown by the writings of Cowles ('01), and in considerable detail by 

 Clements ('05). The discussion by Clements will be particulairly valu- 

 able to the student of avian succession. 



III. THE ^lAJOR AVIAN ENVIRONMENTS. 



As has been seen in the preceding review of the literature on haunts, 

 no comprehensive discussion has been given of the environmental influ- 

 ences or ecological distribution of (extra- tropical) North American 

 birds. Various authors have discussed their geographic distribution, 

 and certain geographic variations have been referred to certain environ- 

 mental influences, but a general ecologic treatment, as contrasted with 

 a primarily faunistic one, has not been made. This is remarkable when 

 we recall the fact that the collections of North American birds are, 

 considering the large area concerned, the best in the world both as to 

 quality and as to quantity (Stejneger, '03). This means that there 

 have been many trained collectors; but what has become of the notes 

 and observations on the environments and conditions of life of these 

 birds, which must necessarily have been known to successful collectors? 

 Part of these observations have been published, and perhaps no one is 

 to blame because more have not; but the point of significance is that 

 we have, in fact, hardly made a beginning in the careful detailed study 

 of the bird environment and its development as a distinct field of study. 

 In common with the remainder of the North American biota, several 

 general principles are known, but they do not appear to be current 

 among ornithologists. 



The following discussion and suggestions on the larger environmental 

 units attempt only an outline of certain phases of the problem, in order 

 to call attention to certain principles which seem useful as a background 

 for the intelligent study of bird habitats and succession. From such 

 a standpoint as this, the dominant influences of given areas and environ- 

 ments are of particular interest and of fundamental value. By focuss- 



