340 DEPTHS OF THE OCEAN chap. 



to definite marine areas, whereas in reality the areas of distri- 

 bution of the different species encroach so upon each other, 

 that a division of this kind is hardly practicable. This is true, 

 not merely of the altering flora of ocean-currents, but also of 

 the attached flora along the coasts and on land. Unless the 

 areas are exceedingly remote from one another, the forms 

 common to the areas usually exceed those peculiar to each 

 area. Cleve's types, on the contrary, have no species in 

 common, and therefore do not record the species in any 

 definite area, but merely group them in accordance with their 

 conditions of existence. If we adopt his principles we can 

 certainly obtain a biological division of the species that is 

 satisfactory in the main ; but when we come to details it will, 

 in some cases, be difficult to decide whether a species is to be 

 assigned to this or to that type. 



Biogeographically, the pelagic alga; may be divided, firstly 

 according to the latitudes in which they are distributed, which 

 is generally tantamount to saying according to their need of 

 warmth and light, and secondly according to their occurrence 

 along the coasts or in the open sea. This latter classification 

 gives us the most distinct boundaries, and we will therefore 

 consider it first. There is a whole series of species which 

 unmistakably belong to coastal waters, and occur there in 

 myriads at definite seasons of the year. Out in the ocean we 

 do not find them, except when salinities or other physical 

 properties indicate that they must have drifted from the coast. 

 iiaeckei. These have been termed neritic on the suggestion of Haeckel. 

 Opposed to them are the oceanic species, which belong to 

 the ocean and float over profound depths, from which 

 occasionally they are swept by the currents into coastal seas 

 and there usually perish. 



Neritic It is possible to imagine various reasons why the neritic 



species. species keep in the vicinity of the coasts. Some may derive 



benefit from the low or fluctuating salinities, which enable them 

 to outstrip the more easily affected forms. Others, perhaps, 

 require the abundant supply of nourishment from the land 

 in order to grow and multiply as fast as such organisms should 

 do. There may be other species, again, whose development- 

 history makes it necessary for them to remain on the bottom 

 at one stage of their existence, like the hydroid medusae and 

 all pelagic young-stages of littoral animals. Most of the neritic 

 algae have a bottom-stage, in so far as they form resting-spores 



