28 THE DEPTHS OF THE SEA 



than 50 fathoms. Below these limits all seasonable 

 variations cease. Summer and autumn, spring and 

 winter, are unknown to the dwellers of the deep ; and 

 the burning sun of the tropical noonday, which heats 

 the surface water to such a degree that the change 

 of temperature from the lower waters to the upper 

 proves fatal to many delicate animals when brought 

 up from the depths, has no effect on the great mass of 

 water below the loo-fathom line. 



Again, in the depths the waters are still. A great 

 calm reigns. The storms which churn the upper 

 waters into tumultuous fury have but a superficial 

 effect, and are unfelt at the depth of a few fathoms. 

 Even the great ocean currents, such as the Gulf 

 Stream, are but surface currents, and their influence 

 is probably not perceptible below 200 fathoms. There 

 are places, as the wear and tear of telegraphic cables 

 show, where deep-sea currents have much force ; but 

 these are not common. We also know that there 

 must be a very slow current flowing from the poles 

 towards the Equator. This replaces the heated surface 

 waters of the tropics, which are partly evaporated and 

 partly driven by the trade-winds towards the poles. 

 Were there no such current, the waters round the 

 Equator, in spite of the low conductivity of salt water, 

 would, in the course of ages, be heated through. But 

 this current is almost imperceptible ; on the whole, no 

 shocks or storms disturb the peace of the oceanic abyss. 



An interesting result of this is that many animals, 

 which in shallower waters are subject to the strain 

 and stress of tidal action or of a constant stream, 

 and whose outhne is modified by these conditions, are 

 represented in the depths by perfectly symmetrical 

 forms. For instance, the monaxonid sponges from 

 the deep sea have a symmetry as perfect as a lily's, 

 whilst their allies from the shallower seas, subject as 

 they are to varying tides and currents, are of every 



