10 THE VOYAGE OF H.M.S. CHALLENGER. 



It must, indeed, be admitted that the presence of a single common form in two widely 

 distant seas — such as the Arctic and Antarctic Oceans — throws a difficulty in the way of 

 admitting different local floras. Thus, if only a single species could, by currents or 

 from any other cause, have migrated in one or more stages from one point to another, it 

 must follow that the general mingling of the various marine genera and species is reduced 

 to a mere question of time. It remains, however, to be proved that in the polar seas the 

 Diatoms that occur, though admittedly belonging to the same genus, are also positively 

 of the same species. On account of our imperfect knowledge and of our limited means of 

 observation, it cannot be asserted that two such frustules do not form two distinct species, 

 though their specific characters may appear to be identical. 



But allowing that some common species do exist in widely separated oceans, it may be 

 safely concluded that, since the complete fusion of all marine floras has not taken place in 

 the course of the thousands of centuries during which Diatoms have existed, it will not 

 happen for many ages to come. 



What, then, is the meaning of setting a limit of any kind to the habitat of marine 

 species 1 In the present state of knowledge no definite information can be offered, and 

 it may be long before the circumstances under which the development of one diatoma- 

 ceous form takes place in a given locality, to the exclusion of another, are fully known. 

 Remembering, however, the great facility with which Diatoms adapt themselves to the 

 most varied conditions of life, the numerous opportunities afforded for the distribution of 

 microscopical marine organisms, and the occurrence of different temperatures at different 

 depths in the sea in the same latitude, it may with some probability be asserted that the 

 extension of species in the sea is determined to a great extent by the temperature of the 

 water. If such were not the case it would not be easy to explain why the Diatoms of 

 temperate seas should not be found in company with glacial forms, while, on the other 

 hand, it must be expected that, apart from considerations of pressure and other physical 

 conditions of environment, the species of the polar seas should be met with in great depths 

 in temperate waters. 



Another circumstance which may have tended to prevent the fusion of local micro- 

 florae into a universal one may here be referred to, having been recently pointed out by 

 Mr John Murray in a preliminary report of observations made on board the Challenger, 

 read before the Royal Society of London. 1 



" In the Southern Ocean," says Mr Murray, " south of Kerguelen, in the Arafura Sea, 

 off the coasts of Japan, New Guinea, North America, and in enclosed bays and river 

 deltas, — in short, wherever the specific gravity of the sea is low from an admixture of fresh 

 water, we have met with very many Diatoms on the surface." 



After examining an abundant supply of materials from the Red Sea I have been led to 



1 On Surface Organisms and their Relation to Ocean Deposits, Proc. Roy. Soc. Lond., vol. xxiv. p. 

 533, 187G. 



