188 C. H. OHtenfeld: 



wliile the extinction in the Tropics of Zostera has not been as com- 

 plete, as is the case with Pusidonia ; and Zof^fera does not show 

 8uch marked signs of waning as it does. 



The younger type of the genus, Z. marina, is distributed along 

 the coasts of Europe, from the Black Sea and the Mediterranean 

 in the south to Lapland (Murman coast) in the north. From the 

 British Isles it has reached the Faeroes and Iceland ; and it is also 

 found at one locality on tlie western side of Greenland. But its 

 occurrence here is, I think, due to accidental transport by man, 

 as it is near places inhabited, onwards to the present time, from 

 the time of the old Xorse colonists.! Along the Atlantic coast of 

 America we find Z. marina, extending from the Gulf of St. Law- 

 rence to Virginia. Quite separated from its Atlantic area is the 

 North Pacific one, which on the American side extends from 

 Southern Califoi-nia to Alaska, while the Asiatic side includes the 

 coasts of Manchuria and Japan. 



Z. marina is tlius distributed along all the coasts of the North 

 Atlantic and tlie North Pacific, but it is wanting in the intermedi- 

 ate area, the Arctic Ocean (with the above-mentioned exception of 

 Greenland). The question now naturally ai-ises : How has it come 

 to liave this discontinuous distribution? The possibility that it 

 originated during a warmer climate in the Arctic sea, and was 

 driven southwaids by a deterioration of the climate, is hardly prob- 

 able. 



The origin of the genus was supposed to be tropical, and it is 

 more probable than Z. niarina also, which is ])y no- means young, 

 in spite of being younger than the Z. nana gioup, originated in 

 a Avarm ocean and migrated northwards; but tlie evidence is not 

 sufficiently clear whethei- tliis home was the Pacific or the Atlantic. 

 As it is not probable tliat the one species originated in botli oceans, 

 a migration from one ticean to the other seems necessary. We have 

 then two ways of migiation : (1) Eithei- it migrated through a 

 sea-connection v.hich dot's not now exist — f'.(j.. through the Carib- 

 bean connection (just as it is pi'obable tliat the ('ari])bean group 

 of sea-grasses did); (2) or, it migrated by way of the An-tic ocean 

 at a time Avhen its waters were Avarm. The latter way of migra- 

 tion seems moie probable from a biological point of view, and is 

 better in accord with tlie present-day c<ui(litions life of the 

 species. 



Much of wliat has hen said liere about tlu' evolution of the sea- 

 gra.sses and tlifir iiiigiations is, of coursi-. only hypothetical, and 



