44 CATALOGUE OF MARINE ALG1E OF WEST INDIAN REGION. 



of the Indian Ocean region, compared with the coast-lines of 

 Eastern, Southern, and Western Australia, and the S. Pacific 

 islands. If we trace the Gulf Stream and the Equatorial current 

 to its source, we travel up-stream round the Cape of Good Hope, 

 and arrive at the coast of Western Australia. Is this great current 

 to be admitted as a factor in solving the problem ? Against it one 

 may hear urged that the Cape total is lower than the Australian, 

 but against this objection is the further fact of the small coast-line 

 of the Cape. 



However this may be, the Indian Ocean region has, both 

 relatively to Australia and relatively to its total flora, surprisingly 

 little in common with the West Indies as regards species. If we 

 take the genera, which are either confined to the tropics or are 

 almost exclusively represented in the tropics, we shall find, speaking 

 broadly, that the genera of marine Algae are the same in the East 

 Indies as in the West, while the species are in a very high 

 proportion different. When we regard these regions shut in by 

 continental areas, and not much less effectively by areas of low 

 temperature in the ocean, there is a strong temptation to rush 

 at once to a conclusion that the tropical genera of Algae are 

 thus proved to be of immense antiquity, while the species are 

 comparatively recent. However convenient such a conclusion 

 might be, it would be rash to assume its truth on this evidence 

 alone. When these oceans have been more thoroughly explored, 

 and their forms more critically studied, it will be time to debate the 

 question. But the evidence is strong enough at present to warrant 

 our briefly considering its position. Admitting the enormous 

 antiquity, in a geological sense, of the continental areas their 

 " permanence," as one writer puts it as beyond question an 

 insuperable barrier to the mingling of tropical marine forms, we 

 may yet ask, Does this hold good of the ocean temperatures ? The 

 changes and variations of climate in the northern and southern 

 hemispheres are admitted by all to have been both far-reaching and 

 of great duration. It is also generally admitted that an important 

 agent hi determining such variations of climate, or, at all events, a 

 constant accompaniment of such, has been change of direction of 

 the great ocean currents. Let us suppose in the past a past by 

 no means so remote as the age of continents a more genial climate 

 in the southern hemisphere, and we at once obtain conditions 

 suitable for the migration and mingling of the marine forms of 

 tropical oceans by way of the Cape of Good Hope. It will be seen 

 that this reduces to (in geological sense) comparatively modern 

 times the latest age during which continental areas formed no 

 insuperable barrier between tropical seas. It may now be said that 

 this argument tends to bring the age of the tropical species down to 

 comparatively recent times. It may bear that interpretation to those 

 who choose to make it, but I would emphatically point out that it 

 by no means forces us to make it. I have dwelt at this length on 

 this aspect of the question because it appears to me that we have 

 here one of the most interesting points in the distribution of plants, 

 and one on which further labour may be hopefully expended, I 



