Indian Ocean, and the Cape of Good Hope. 67 



than 2 species only to the genus, the West Indies and Australia average 

 rather more than 5 and less than 5 respectively. I estimate that the 

 north temperate Atlantic yields an average of about 4^ species to the 

 genus, and the difference between this and 3 species per genus found 

 at the Cape is to be attributed primarily to the short coast line of the 

 Cape, and in a less degree to its Algae being less known. The calculation 

 of such averages and proportions appears to me to be justified only when 

 applied to the whole flora, and becomes more dangerous and apt to mislead 

 when applied to portions of it, since particular groups in all the floras have 

 been subjected to unequal treatment by collectors and describers, and we 

 may perhaps trust to these personal errors neutralising each other when 

 the complete totals are compared. 



The warm Atlantic and Cape have 85 genera and 114 species in 

 common, while the Indian Ocean and Cape have 86 genera and 89 species 

 in common. That the number of genera in common should be so nearly 

 exactly similar is interesting, and to discover whether they are the same 

 genera in many cases it is only necessary to turn to the last table, where 

 the Algae common to all three regions are given to find that 72 genera are 

 are common to all three. Some years ago I hazarded the speculation 

 that, while the genera of the tropical Atlantic and those of the Indian 

 Ocean were largely the same, the species were, in a high proportion, 

 different.* We can now see that they have no less than 103 genera in 

 common out of a total of 139 occurring in the Indian Ocean and 162 

 in the warm Atlantic. They have certainly more species in common, 

 viz., 173, but these must be considered relatively to the two totals of 

 514 in the Indian Ocean and 859 in the warm Atlantic, when my expecta- 

 tion will appear to be fairly borne out. Nevertheless, I confess to having 

 anticipated an even greater diversity of species. That the absolute 

 number of genera occurring at the Cape should be by two greater than 

 those of the Indian Ocean completely puzzles me. I cannot fully 

 account for it on any theory. While the number of species in common 

 between any two of the floras is greater than the number of genera 

 (though in one case only three more), the number of species, as might be 

 expected, in common to all three vi/., 59 is less than the genera viz., 72. 

 Again I should have expected to find relatively fewer species in common. 



When one comes to analyse these totals, the process must be carried 

 on in a more guarded fashion. One expects, as shown above, to find 

 fewer species to the genus at the Cape than in the tropical floras, but one 

 hardly expects to find that the genera of Floridece at the Cape are by five 



* Catalogue of Marine Alga of the West Indian Region. 



