3i8 DEPTHS OF THE OCEAN chap. 



floating existence affects the whole structure of these alg^, 

 though it is not always carried out to the same degree in the 

 different genera and species. If we examine into their distribu- 

 tion we shall find that no particular region is distinguished by 

 specially well -equipped species. Genera with the greatest 

 numbers of species have their representatives in both the 

 warmest and the coldest areas of the sea, and no essential 

 difference in the development of their suspension-apparatus is 

 to be found between the species of ChcEtoceras and Rhizosolema 

 which live near the confines of the polar sea, and their relatives 

 in the tropics. The greatest abundance of forms is to be met 

 with in coastal waters, where, too, the majority of the species 

 have their home. I shall return later on to the special biology 

 of these coast-forms. 



Many species of diatoms show variations indicating that 

 within certain limits the algae can adapt their floating power to 

 the demands made on them. Their tendency to sink increases 

 with a rise of temperature, and decreases with an increase of 

 salinity. It is not alone the specific gravity (density) of 

 sea-water that is here the determining factor; no doubt we 

 must bear specific gravity in mind also, but its variations are 

 comparatively small. Ostwald has shown that the internal 

 friction or viscosity of sea-water is the most important con- 

 sideration, and this diminishes with an increase of temperature. 

 Other things being equal, sea-water at 25° C. offers only half the 

 resistance that it would at freezing-point. Salinity, on the 

 other hand, is of less account. A rise of i per cent in the 

 salinity will produce no more than an increase of 2 to 3 per 

 cent in the internal friction, and as salinity in the open sea is 

 subject to what are after all quite inconsiderable variations, it 

 follows that it is really temperature which indirectly affects the 

 development of the suspension-organs. In areas of the sea 

 where there is a big difference in temperature between summer 

 and winter, we find a number of species with distinct summer 

 and winter forms, that have sometimes even been supposed to 

 belong to totally different species. And the same variation 

 occurs also in species with a wide distribution, the warm-water 

 types corresponding to the summer forms, and the cold-water 

 types to the winter ones. The summer forms have usually 

 thinner cell-walls, and a more slender structure ; their excess 

 weight appears to be reduced, though at the same time 

 their surface is comparatively larger. As, however, diatoms 

 vary greatly in their dimensions throughout their life-cycle, 



