390 BULLETIN OF THE 



cide with isothermal lines. These isothermal lines, however, are not 

 so often the yearly isotherms as those of particular seasons. While 

 some writers have considered isocrymal lines as those having the 

 greatest amount of limiting influence, as Dana has supposed to be the 

 case with marine animals,* and as may be true in the case of plants, 

 and possibly also of some terrestrial animals, f the mean temperature 

 of the breeding season must necessarily more affect birds, especially the 

 migratory species, than that of any other part of the year, or than the 

 mean annual temperature. Isotherals hence most nearly coincide with 

 the lines limiting the distribution of birds in the breeding season, and 

 also the ornithological faunae, since the majority of the species in the 

 region now under consideration breed almost exclusively during the 

 summer months, .and mainly in June and July. Some breed in May, 

 and a few of the rapacious birds in April, and even in March, but they 

 are the exceptions to the general rule. The isotheral lines are hence 

 adopted in the present essay in giving the boundaries of the ornitho- 

 logical faun se. t 



Owing to the imperfect state of our knowledge of the summer dis- 

 tribution of the birds of North America, the present attempt at a defini- 

 tions of the interior, as well as forming the limit of steam navigation on the rivers of 

 the lower Atlantic States, forms also the dividing line between the fauna? of the coast 

 and thoso situated next to them in the interior, although having an altitude of generally- 

 less than three hundred feet. The rise from the succeeding plateau to the more abrupt 

 slope of the Appalachians forms likewise the boundary between the second and third 

 tiers of faunae in the Atlantic States. The terrace forming the northern boundary 

 of the tertiary deposits of the Gulf States, and of the lower Mississippi Valley generally, 

 coincides likewise with faunal boundaries, as do similar slight changes in elevation 

 elsewhere. 



* See Report on the Crustacea collected by the U. S. Expl. Expd. under the com- 

 mand of Captain Wilkes, Vol. II, p. 1452. 



t There must; however, be many exceptions, since in cold climates many mammals 

 and all reptiles, as well as a large proportion of the molhisca and insects, hibernate, 

 and thus are to a great degree (especially the reptiles) beyond the influence of excessive 

 cold. In regard to plants, also, their northward range seems to be limited more by the 

 amount of heat in summer than by the cold of winter, particularly in the case of annuals. 

 As soon as the sum of the heat ot summer is diminished to such a degree as to be 

 insufficient to mature the plant, or to allow it to ripen its fruit, whether an annual, a 

 shrub or tree, it must at that point cease to propagate, and there find its polar limit. 



} Professor A. E. Verrill states that he has found the "boundaries between the 

 Canadian and Alleghanian Faunas" to be " coincident with a line which shall indicate 

 a mean temperature of 50° Fahrenheit during the months of April, May, and June." 

 Proc. Dost. S"c. Nat. Hiti., Vol. XII, p. 260, May, 1S66. 



