currents. 



558 DEPTHS OF THE OCEAN 



"Michael Sars " in 1910 tests were made at various depths, 

 and it was found that the Hght was far stronger south of the 

 Azores than in the northernmost portion of the Atlantic at 

 corresponding depths. But whether light is in itself sufficient 

 to explain the different vertical distribution of a species in 

 different marine areas, or whether there are other contributing 

 factors, are matters yet to be decided. So far the question has 

 not been sufficiently studied. 

 Effect of The animals of the ocean-floor owe their distribution mainly 

 to the agency of currents, since these serve to transport their 

 pelagic larvae, and perhaps also carry along full-grown bottom- 

 forms like the amphipods and most of the prawns, which creep 

 almost as much as they swim. It is through transportation of 

 larvae that the Norwegian Sea acquired most of its southern 

 forms, and to this day these forms are still being disseminated 

 in similar fashion throughout its component parts. We must 

 bear in mind that most bottom-animals are attached, or, if 

 we except a few crustaceans, very limited in their locomotion, 

 and that consequently distribution by direct migration is all 

 but impossible. The distribution of larvae is subject to 

 physical laws, and is dependent on the occurrence of the adult 

 animals, and on the hydrographical conditions that prevail. 

 Larvae of arctic forms which inhabit only polar areas will, as 

 a rule, only be transported by polar currents, so that the 

 bottom they will reach, when their development is completed, 

 will lie within the arctic region. In the same way the species 

 belonging to Gulf Stream areas will be retained in boreal 

 waters. 



In addition to the two main currents of the Norwegian 

 Sea there are several others consisting of blended layers, such 

 as mixtures of the Gulf Stream, polar water, coast water, 

 North Sea water, and bank water in various combinations. 

 Probably every one of these plays its own particular part in 

 distributing the larvae, and consequently the bottom-animals, 

 but we do not yet know to what extent. It seems absolutely 

 certain, in view of what we have learnt regarding pelagic 

 animals, that the larvae in an area bordering on two currents 

 may be swept away by one or the other, and so conveyed to 

 a strange area. This, I fancy, explains why a coast-form like 

 our common sea-urchin, Echinus esciilentits, may be exceptionally 

 met with in deep water out in the North Sea and Atlantic, where it 

 succeeds in existing as a somewhat different variety. The occur- 

 rence of the arctic amphipods, Epimeria loricata and Aca7ithozone 



