Geographical Bistrihution of Sea-Grasses. 1 89 



may be wrong. Still, it seems to me that the existing facts of the 

 distribution of the genera and species make deductions of that 

 kind allowable. They are a help in gaining an idea of how these 

 interesting remnants of the older types of Flowering Plants at- 

 tained their present areas of distribution. 



The peculiarities of distribution of the sea-grasses are not with- 

 out parallel. Amongst the Marine Algae similar cases have been 

 reported, (rtorge Murrai/ (l(S7-3) has pointed out that tliero is a 

 g:reat resemblance between the algal flora of the C'ari))bean Sea 

 and that of the Indopacific region. He supposes it to be explicable 

 by a migration by way of the Cape. A. Svedel/iis (1906) agrees 

 with Murray as to the great resemblance of the floras, which he 

 has himself studied, especially in Canhrjja (surely a very old type); 

 but his explanation necessitates a water-connection where the 

 Isthmus of Panama now is. This is essentially the same explana- 

 tion as that I have offered with regard to the origin of the Carib- 

 bean sea-grasses. 



///. Summary. 



1. The Marine Flowering plants, the sea-grasses, belong to two 

 families of the old monocotyledonous cohort, the Helohiecip. Tliey 

 are reduced and specially adapted ancient types. 



2. With one exception (the monotypic Enhalus). the genera all 

 occur in the Indopacific and the Atlantic regions. This distribu- 

 tion indicates that the origin of the genera goes back to a time 

 when the relation of sea and land was different from that now — • 

 viz. to the earlier part of the Tertiary period. 



3. The dispersal of the sea-grasses is restricted and slow. It 

 is mainly due to sea-currents, but the seeds are not able to float, 

 and detached pieces of rhizomes do not live long. 



4. The 8 genera contain altogether only 30 species, which can 

 be arranged into 8 groups, according to their geographical distri- 

 bution — viz. : (a) 4 Tropical: Indopacific, Malayan, East-African 

 and Caribbean ; (b) 2 W arm-temperate (or sub-ti'opical) : Austra- 

 lian, Mediterranean; and (c) 2 Temperate: North-Pacific and 

 North-Temperate. . 



5. The Indopacific group contains the majority of the species, 

 especially when we include the Malayan and East-African groups 

 as sub-divisions (13 species). The parent home of the sea-grasses 

 lies within the geographical area of this group. 



6. The Caribbean group (6 species) has 4 species, each of which 

 is closely allied to an Indopacific one, and it is probable that the 



