COMPREHENSIVE LIST 



covering, as a whole, the entire state; but a certain 

 number of species have a more or less restricted breed- 

 ing range, one set being confined to the more northern, 

 the other to the more southern portion of the state. 

 Some of the Transition Zone species occupy, during 

 the breeding season, only the northern portion of that 

 zone, others most of that area, while others invade, to 

 a greater or lesser extent, the Upper Austral Zone. 

 In like manner some species are restricted to the more 

 southern portion of the Lower Austral Zone, while 

 others extend, in some cases at least as recent invaders, 

 farther northward, some of them nearly across the 

 Upper Austral Zone, or even to within the edge of 

 the Transition Zone. The Upper Austral Zone, there- 

 fore, is an area of overlapping of Transition Zone and 

 Lower Austral Zone species, there being none peculiar 

 to the area. In reality, the life-zones, as generally 

 understood and accepted, are more or less indefinite 

 and even unstable. As an able writer on the subject 

 has remarked, "the present aspects of dispersal are 

 a result of past conditions a fauna is the expression 

 of a certain adjustment between organisms and their 



environment, and the most important direct factor 



in any environment is the nature of the vegetation 

 which is conditioned by soil and by the climatic factors 

 of heat and moisture, and that our so-called faunas 



in reality represent a somewhat temporary state 



of groups of species in relation to breeding areas, and 

 the more or less arbitrary boundaries of these faunas 

 represent our knowledge only of the present conditions 

 of distribution. " c 



That the breeding range of some species of birds 

 has been subject to mutations of very considerable 

 extent within the memory of persons now living is 

 a well-known fact. The practical disappearance of 

 the Dickcissel from the entire country east of the 

 Alleghenies is a case in point. Here in Illinois three 

 notable examples may be cited : the song-sparrow, 

 which in seven years extended its breeding range one 

 hundred miles farther southward in the Wabash Valley 

 (from Paris to Grayville) ; the House Wren, which 

 previous to 1869 was absolutely unknown in south- 

 eastern Illinois, where it has since become abundant 



24 



