290 Capt. F. W. Button on some of the 



the marine plants^ and, consequently, the Petrels, approach 

 much nearer the equator in the Antarctic than in the Arctic 

 seas. On the other hand, although the number of individuals is 

 immense, the species are few, which is, doubtless, owing to the 

 uniformity of the conditions under which they live. 



It is very curious to note that most of the species of the Pro- 

 cellariidcB inhabiting the northern hemisphere have " analogues/' 

 or closely resembling species, in the southern hemisphere. For 

 instance, the Albatros of the North Pacific Ocean, Diomedea bra- 

 chyura, Temm., very closely resembles D. exulans, L., although it 

 is undoubtedly distinct from it. Procellaria glacialis, L., and Pro - 

 cellaria pacifica, An dub., again, are nearly related to Procellaria 

 glacialoides, Smith ; Puffinus cinereus (Gmel.) and Puffinus major, 

 Faber, to Pi'ocellaria hasitata, Licht, {nee Kuhl) ; Puffinus anglo- 

 rum, Temm., and Puffinus obscurics (Gmel.) to Puffinus assimilis, 

 Gould ; and the Stormy Petrels of the northern seas, with the ex- 

 ception of Thalassidroma leachi, Temm., diiFer from those of the 

 southern seas only in some minor points of plumage, Thalassi- 

 droma oceanica (Kuhl) being, as far as I know, the only Petrel 

 common to both hemispheres. But while most of the northern 

 Petrels have representative species in the southern hemisphere, 

 many of the southern ones find no analogue in the northern 

 hemisphere — e. g. Procellaria gigantea, Gmel. ; Daption capensis 

 (Linn.) ; Prion vittatus (Gmel.) ; Pelecanoides urinatrix (Gmel.), 

 &c. These facts make it appear probable that the northern 

 species crossed over the equator from the southern hemisphere, 

 perhaps during the glacial period, and, having been isolated ever 

 since, have varied somewhat from the parent forms ; and the fact 

 that, in the three cases I have mentioned, two distinct species in 

 the northern hemisphere are closely related to one species in the 

 southern hemisphere, points also to the same conclusion. The 

 great extent of land in the northern hemisphere will probably 

 explain why the genus Puffinus, whose habits are the least oceanic 

 of any (except Pelecanoides, which does not appear to have crossed 

 the tropics), has been so largely developed in those seas, while that 

 of Procellaria is restricted to two closely allied species. The only 

 two species of Procellaria at present known to inhabit the tro- 

 pical parts of the Pacific are also each represented in the Southern 



