CLIMATIC INFLUENCES. 39 



where, consequently, as baa already been stated, tbe differences 

 between the temperature of night and day are excessively marked. 

 In the desert region of Western Asia — Persia and the Valley of the 

 Euphrates — the bird ranges or ranged as far north as about the 

 thirty-fifth parallel of latitude, and, indeed, it is not exactly im- 

 probable, as has been maintained by Vamb6ry,'* that even at the 

 present day it exists in limited numbers along the shores of the 

 Sea of Aral, in about the forty-fifth parallel, or what would cor- 

 respond to the position of the southern portion of the State of 

 Maine. In the case of this family — Struthionidae — we also notice 

 the singular fact, analogous to that which has been observed in 

 relation to the distribution of the Camelidae, that the only repre- 

 sentatives of the group other than Struthio (the ostrich proper), 

 constituting the American genus Rhea, are birds belonging almost 

 strictly to the temperate regions, then- range extending from Pata- 

 gonia to the southern confines of Brazil. The parrots (Psittaci) 

 may be considered to be preeminently tropical birds, the vast ma- 

 jority of the species being included in a zone bounded by the 

 thirtieth parallel on each side of the Equator, but yet it may be 

 doubted whether this limitation does not depend more upon the 

 nature of the food-supply than upon the character of the climate. 

 In South America a species of Conurus extends its range as far as 

 the Strait of Magellan, and in the Macquarie Islands, in the South 

 Pacific, representatives of the family are met with as high as the 

 fifty-fourth parallel of latitude, corresponding to a position removed 

 by only six degrees from the southern extremity of Greenland. 

 Wallace probably justly refers to the "almost universal distribu- 

 tion of parrots wherever the climate is suflRciently mild or uniform 

 to furnish them with a perennial supply of food." '^ 



But while in numerous, and perhaps the majority of, instances the 

 limitation of animal groups to certain geographical regions is de- 

 pendent more upon the physical character of the immediate environ- 

 ment and the nature of the food-supply than upon particular con- 

 ditions of climate, yet it cannot be denied that in very many cases 

 climate appears to exercise a paramount influence upon distribution. 

 This influence is frequently considered to be nowhere more forcibly 

 illustrated than in the migration of birds, both as regards the 

 northern species and those inhabiting the southern climes. That 

 the climatic explanation of the phenomenon of bird migration is a 



