Vor, Diy Pr it) LOOMIS—A REVIEW OF THE TUBINARES 13 
The ancestral line of the Tubinares extends back into the 
Miocene Period, and the causes affecting the distribution of 
the group involve past conditions as well as present ones. 
The preponderance of species in the Southern Hemisphere 
and on the Pacific Ocean, the restriction of the Pelecanoididze 
and certain other groups to Southern seas, and the dual hemi- 
sphere distribution of the Wedge-tailed Shearwater and 
White-faced Petrel are not explained solely by existing con- 
ditions. There is an historic background. In the remote 
past Tubinarine species became established in their habitats 
and have been able to hold them against all competition. 
Their success is perhaps due chiefly to the isolated character 
of their breeding stations, where predaceous land mammals 
are absent and food is plentiful. The discontinuous distribu- 
tion of Harcourt’s and Bulwer’s petrels ceases to be an 
enigma when viewed from the standpoint of a water way 
between North and South America, which geologists tell us 
existed as late as the Miocene Period. 
The fact is not lost sight of that temperature is a factor 
in the geographic distribution of birds. The apparent restric- 
tion of certain Tubinares in the Southern Hemisphere to the 
zone lying approximately south of the isotherm of 15° C. for 
January perhaps indicates a temperature control. It should 
be borne in mind, however, that temperature on the ocean is 
affected not only by latitude, but by periodic winds, warm 
and cold currents, and apparent upwelling of cold bottom 
water, and it may be that these persistent phenomena, in- 
‘stead of exercising temperature control, serve merely as 
boundaries, marking the frontiers of habitats established in 
ages long past. 
Barren areas in the sea and areas having a food-supply 
also determine distribution. In the former bird-life is want- 
ing and in the latter it may abound, as in the south polar 
seas, where there exists an extraordinary abundance of food 
in summer and autumn.* 
1Cf. Clarke, Ibis, 1907, pp. 328, 329. 
