2G1 [VerriU. 



Valley of the INIississippi as far as St. Paul, and perhaps along the 

 Red River of the North, but in that region there is again a lack of 

 data for accurate determination. Where this line reaches the Middle 

 Ornithological Province of the North America, so well defined by- 

 Prof. Baird,* and -whether corresponding divisions by temperature 

 exist in the Mddle and Western Provinces, are questions that can 

 only be determined by more extended observations. 



In like manner the line of 65° mean temperature, during the same 

 months, coincides with the boundary between the Alleghanian Fauna 

 and that of the Southern States, or Louisianian Fauna. This line com- 

 mences on the Atlantic coast, near Portsmouth, Virginia, and passes up 

 the Valley of the James River, thence to the vicinity of Gordonsville, 

 Va., and westward till it reaches the eastern slope of the mountains of 

 central Virginia, when it passes far to the south through the Carolinas 

 and Georgia along the mountain region, which it finally crosses, 

 and then turns again to the northward along its western side. It 

 passes through central Tennessee, north of Memphis, and through 

 Kentucky, bending northward up the valley of the Ohio nearly to the 

 vicinity of Cincinnati, and up the Mississippi to an undetermined 

 distance beyond the mouth of the Ohio. West of the Mississippi the 

 line has not been satisfactorily determined. 



Again the temperature of Southern Florida, during the same time, 

 is about 80°, and there we find a few birds that do not extend farther 

 northward, and one that is peculiar to that region ; thus indicating the 

 commencement of another fauna, which may, perhaps be considered 

 as a part of that of the West Indies. 



From this remarkable coincidence between this system of lines of 

 temperature of the months of spring and early summer, with what 

 had been already observed in the actual distribution of birds, we must 

 necessarily infer that they are chiefly influenced so far as latitude is 

 concerned, by the temperature of the breeding season. Therefore we 

 should expect that in all other countries, the tropics, perhaps, excepted 

 the same law would hold good. W^hether a similar law controls the 

 distribution of Mammalia, Reptiles, Insects, etc., can only be deter- 

 mined by farther investigation. Prof Dana, in his great work on 

 the Crustacea of the United States Exploring Expedition, estab- 

 lished the law that the distribution of Crustacea in latitude is con- 

 trolled by the mean temperature during the winter months, which 

 is evidently a law analogous with what we have observed in birds. 



It has also been observed by several botanical writers, that the 

 distribution of vegetation is controlled by the temperature of the 

 summer months, — the mean temperature of June, July, August, 

 and September, being usually considered the most important for this 



* American Journal of Science, Jan., 1866. 



