424 BULLETIN OF THE 



it as the result of a general law, and to remove it from the category 

 of genuine specific distinctions.* 



X. Tlie number of species which breed in the American Arctic 

 Fauna appears to be not far from sixty. In the Hudsonian Fauna the 

 number is increased to upwards of one hundred and fifty, in the Cana- 

 dian to probably about one hundred and sixty. In the Alleghanian 

 the number is nearly one hundred and forty; in the Carolinian about 

 one hundred and thirty-five; in the Louisianian about one hundred and 

 thirty. The Hudsonian and Canadian Faunae hence have a greater 

 number of species, in areas of the same extent, and probably a far 

 greater number of individuals, than the Carolinian and Louisianian 

 Faunae. In respect to the number of individuals, it is evident that 

 this must result, in consequence of the hordes of wading and swim- 

 ming birds, of thrushes, sparrows, and Sylvicolidce that pass through 

 the southern and middle districts of the Eastern Province to breed 

 in its boreal portions ; few of the species that breed at the southward 

 being there as numerously represented as are scores of species that 

 breed exclusively at the northward. Taking the whole number of 

 species found at particular localities during the year, there is a con- 

 stant increase in number to the southward, the increase, however, 

 being less rapid from the southern boundary of the Canadian Fauna 

 southward than from that point northward.! There is also a steady 



* In addition to the list of examples of this variation already cited in Part III of 

 this paper, the following may be added as marked instances: Reyulus satrapa, Pacific 

 coast specimens forming the variety olivaceus Baird ; Troglodytes hyemalis, Pacific coast 

 specimens forming the variety pacificus Baird; Cistothorus palustris, Pacific coast speci- 

 mens forming the variety paludicola Baird; Helminthophaga celata, Pacific coast speci- 

 mens being, according to Professor Baird, " much brighter and clearer yellowish be- 

 neath and olivaceous above," than those from the interior of North America ; Myiodioctet 

 (= Wihonia) puiilla ; Ilirundo bicolor ; Collwrio excubiloroides (= C. ludovicianus), Pacific 

 coast specimens being darker than those from the Mississippi Valley, and much darker 

 than those from the Plains {Baird); Ctrpodacus purpureas, Pacific coast specimen!) 

 being darker and forming the C. californicus of Baird : Mdospiza melodia, the darker 

 Pacific coast specimens forming the M. Ihermanni Baird, etc. 



t There seems to have been no exhaustive list published of the birds occurring at 

 any locality north of the Alleghanian Fauna. Dr. Richardson's list is the largest, and 

 gives two hundred and thirty-eight as the whole number known in 1831 to inhabit 

 British North America north of the Canadas and east of the Pocky Mountains. Prob- 

 ably this number, and even more, may occur at a single locality on the Saskatchewan; 

 but probably not more than two hundred and sixty or seventy. Three hundred and 

 ten have been detected in Massachusetts, including those of irregular and very rare 



