68 Marine Floras of the Warm Atlantic, 



more numerous than in the warm Atlantic, and by 15 more than in the 

 Indian Ocean. There no less than 95 genera of P"loridece at the Cape, with 

 295 species, while the 90 of the warm Atlantic contain nearly 200 more 

 species ! Matters are much the same in the case of the Phceophycece, and 

 we have to come to the CJdorophycece to redress the balance in the case of 

 the warm Atlantic. They just fail to bring it level in the case of the 

 Indian Ocean. It has been remarked above that the genera which the 

 two tropical floras have in common with the Cape are almost identical in 

 number. The analysis shows that the figures are very steady, viz., 58 each 

 of FloridecB, 14 and 15 of PJuzophycea?, 11 each of Chlorophycece, and 2 each 

 of Protophycecz. The table shows the tropical character of such a group as 

 the Siphonece very markedly. There are 99 species in 23 genera in the warm 

 Atlantic, 72 species in 16 genera in the Indian Ocean, and only 20 species 

 in 7 genera at the Cape. It is interesting to observe that the whole of 

 the 16 genera of Siphonece in the Indian Ocean are represented in the 

 warm Atlantic. It has no peculiar generic type of its own in this tropical 

 group. While the genera of this tropical order are thus practically 

 identical, the species are in a very high proportion different. Only 29 are 

 possessed in common out of the two totals of 99 and 72. In the com- 

 parison of the two tropical floras there is the coincidence that the 

 genera and species of Siphone<z agree exactly in numbers, viz., 16 and 29, 

 with the total of all the PliceopJiycece a thing without significance, how- 

 ever. But I need not go farther into the details of the table. 



The interest that is attached to the above comparison is mainly this. 

 We have here two tropical marine floras cut off from each other by a 

 permanent continental area, and communicating only via the Cape. 

 That these floras have been periodically mingled at the epochs of warmer 

 climate at the Cape seems a reasonable conclusion with regard to a group 

 of such antiquity as the Algae, and the proportions of species in common 

 and genera in common between the different regions, and among all 

 three may have a significance in this respect to students of distribution 

 (cf. the totals of Sipkone<z> a peculiarly tropical order). I have 

 elsewhere* commented on the fact that, ' while in the Arctic and 

 Australian regions the Ph(zophyec<z far outnumber the ChloropJiycece, in 

 the tropical West Indian flora the proportion is very markedly reversed, 

 and the green Algae outnumber the olive-brown. One is tempted to put 

 this down to the strong illumination of the tropical sea, but another 

 reason is to be found in the fact that a number of the Antilles richest 

 as regards Algae are subject to irruptions of fresh and brackish water 



* Trans. Bid. Soc. Liverpool, vol. v., p. 178. 



