Geographical Distribution of Sea-Grasses. 18o 



The two gioups. II. and III., are naturally placed as ^slll)divi^S 

 fiions of group 1.. and i-epresent two lines of differentiation finiii 

 the main l)ody of species, with its wider distiihution. 

 They show species, which have not yet attained their limits 

 of distribution, as it seems hard to understand that a species dis- 

 tributed along the shores of the lied Sea and the western side of 

 the Indian Ocean, should not be able to live along the Asiatic and 

 -Malay coasts, and rice versa . The absence of llalopJtila spiniilosa 

 from the Indian Ocean, and that of //. xtipulacea and C i/itiodocea 

 viliata from the Malay Archipelago, are further indicative of the 

 slow rate of niigration of the sea-grasses. These Avell characterised 

 species must have existed as such for a long time, probably as long 

 as a connection between the Indian and the Pacific oceans existed. 



IV. The Caribbean group. 



Halophila Aschersonii. 



Halophila Engelmanii. 



Halophila Baillonis. 



Cymodocea manatorum. 



Diplanthera Wrightii. 



Thalassia testudinum. 



This group contains 4 genera with 6 species. Four of these (one 

 of each genus — are so nearly related to 4 species of the Indopacitic 

 group, that — at least, in some cases — the characters distinguishing 

 them from one another are quite slight. 



The 4 pairs of species are the following : — 



indopacific : 



Halophila decipiens. 



Thalassia Hemprichii. 



Cymodocea isoetifolia. 



Diplanthera uninervis 



Caribbean : 



Halophila Baillonis. 



Thalassia testudinum. 



Cymodocea manatorum. 



Diplanthera Wrightii. 



I think it is advisable to draAv the conclusion that these 4 pairs 

 originated from 4 parent species, which were widely distributed 

 in the tropical seas, and that the present differentiation was sulj- 

 sequent to an alteration in the extension of sea and land — in otiier 

 words : the Caribbean species arose in their present area from 

 ancestors which came to the Caribbean Sea from the Indopacific 



