OBSERVED IN WESTERN IOWA. 489 



It is hence to be much regretted that competent observers should still give us merely nom- 

 inal lists, unaccompanied by notes on the relative abundance and season of occurrence of 

 the different species, and that lists of collections derived from little known countries should 

 be published, as too frequently happens, without stating, when known, the date and exact 

 locality of capture of the specimens. Observations on thirty or forty of the species most 

 common, made in the breeding season, at a hundred or more different localities in the 

 States east of the Mississippi, would enable us quite definitely to indicate the different avi- 

 faunal areas of this region ; and not till then, or at least till our data are vastly multiplied, 

 can we arrive at wbolly satisfactory conclusions. This number of complete lists would be, 

 of course, far better, and if extended, with the same minuteness, througbout the continent, 

 would afford us not only exact data on faunae and the general laws of distribution, but, of 

 course, would give us the means of determining the exact range of each species, at all the 

 different seasons. Hence we deem no apology necessary in bringing forward the following 

 lists of birds observed in the breeding season, though in some cases so incomplete. 



Tbe distribution of marine animals, from investigations made by Dana, Forbes, and others, 

 seems greatly dependent on temperature. Prof. A. E. Verrill 1 has also pointed out the close 

 coincidence of the boundaries of tbe Canadian and Alleghanian faunae, as generally received, 

 with the line of mean temperature of 50° F. for the months of April, May, and June ; at the 

 same time urging that the distribution of birds in latitude is chiefly influenced by the tem- 

 perature of the breeding season. From the coast westward to Lake Michigan, the almost 

 exact accordance of the southern limit in the breeding season of several species of the 

 Canadian fauna, and of the northern limit in the same season of several species of the Al- 

 leghanian with this line, following its southern bendings in crossing mountain ranges and 

 retreating with it northwards as it follows up the river valleys, is most noteworthy, and 

 seems to point to a definite and intimate connection between the two phenomena. In other 

 cases, 2 and in other classes of animals, we have, even from our present scanty data, traces 

 of a similar correspondence ; so positive, indeed, as to lead us to infer a general relation be- 

 tween the limitation of species in latitude and altitude and temperature. On the other 

 hand, it seems equally evident that other physical causes are the governing forces in their dis- 

 tribution in longitude ; but that these are, in fact, mainly climatological, or such as are more or 

 less directly referable to them. The influence of humidity is readily apparent, and so is 

 that of* the vegetation, as exhibited in forest and forestless regions in the United States — 

 conditions plainly the result of climatic causes. 



It is evident that provinces, faunre, and flora?, however strongly marked as a whole, have 

 not abruptly defined boundaries, but shade into each other so gradually as to render the de- 

 termination of their limits from present data but provisional at best, since probably no two 

 species are quite alike in their adaptations, though a certain proportion in a given district 

 coincide more nearly than the others, forming a group characteristic of the region. Through 

 the recognition of such groups, the eastern portion of the North American continent was 



1 Proc. Bost. Soc. Nat. Hist. Vol. X, p. 260. trnpkanes nivalis occur on the slopes of the snow-crowned 



2 It is well known that several species of the Canadian fauna, mountains of southern Mexico, as well as on the more northern 

 as Junco hyemalis, Zonotrichia leucophrys, Pinicola enuclealor table-lands, where altitude produces the isotherms under which 

 (P. "canadensis"), and others, are found in the higher parts the species are found in the northern and alpine portions of 

 of the Appalachian ranges to their southern termination in New England, hundreds of miles to the northward. 

 Georgia, while P. enuclealor, Curoiroslra americana and Plec- 



MEMOIBS BOST. SOC. NAT. HIST. Vol. I, l't. 4. 121 



