554 DEPTHS OF THE OCEAN 



shells of Mytiliis edit lis, Littorina liitorea, and Cyprina islandica, 

 all boreal forms requiring a higher temperature and not living 

 there now. Again, in northern boreal areas there are sub- 

 fossil deposits of molluscs which require greater warmth than 

 .generally prevails in the boreal region {Tapes deaissatiis in 

 Denmiark, Isocardia cor in Norway, etc.), and it is held in 

 some quarters that they could only have existed there when 

 the temperature of the sea was higher. 



Without criticising this theory, I should like to point out 

 that we ought not always to take these finds of sub-fossil shells, 

 belonging to species no longer inhabiting the adjoining seas, 

 as evidence that great hydrographical changes have necessarily 

 taken place in these areas. Tapes deaissahts, for instance, which 

 is now quite extinct along the coast of Denmark, is still to be 

 found at various places along the west coast of Norway, from 

 Bergen down to the south coast, but only in restricted localities 

 where there are special natural conditions, that is to say, in 

 shallow, well-sheltered, sandy bays, dry at low water, but afford- 

 ing full access to the salt water of the sea. These bays differ 

 greatly from the " pools," which have a layer of fresh water 

 at the surface and a muddy bottom smelling unpleasantly of 

 sulphuretted hydrogen, but one feature they do possess in 

 common, namely, that the sun raises their temperature consider- 

 ably above the normal, so much so, in fact, that I have sometimes 

 recorded 23"' or 24° C. in the shallow water of these " Tapes 

 bays " during the summer. Beyond question this high summer 

 temperature, in combination with favourable bottom-conditions 

 and the salt water, enables Tapes deciLSsatus to thrive, and, 

 what is still more important, to reproduce itself It is not 

 difficult to imagine that these rather limited localities may have 

 been silted up, or cut off from the inflowing of salt water in 

 some way or other, thus giving rise to sub-fossil deposits of Tapes 

 shells. Nevertheless, in the case of boreal forms found fossil 

 or sub-fossil in arctic areas, it seems to me that the warmer sea- 

 water theory is the only reasonable one, since there is nothing 

 to indicate that other important factors have been instrumental 

 in their extinction. 

 Effect of It is important to ascertain how changes of temperature 



*^^^"^Suue ^ff^ct ^ species, whether they influence chiefly the development 

 u^i^n animal and growth of the young stages or the full-grown animals through 

 ''^'^" other physiological processes. This question has not been 



deeply studied, though we have acquired sufficient knowledge 

 to enable us to draw one or two conclusions. We know, for 



