778 



DEPTHS OF THE OCEAN 



the 30th meridian west (Fig. 567). We see that the water- 

 layer limited by the isotherm of 10° C. is relatively thin in 

 proportion to the depth of the ocean. The genuine warm- 

 water layers with temperatures exceeding 15° C. reach only 

 to 30° south and north, and are only 200 to 300 metres 

 thick. The whole layer above 10° C. has a thickness 

 varying between 300 and 700 metres (or between ^^ and ^ 

 of the depth of the ocean). Now it was only a part of this 

 small layer which was examined by Hensen's expeditions, 

 and consequently the results must necessarily be incomplete. 



Lat.S 



Equator. 



10° rf 10° 



Lat.N. 



50° 60° 



2500 



Fig. 567.— Distribution of Temperature in the Atlantic along the thirtieth 

 Meridian of West Longitude. (From Schott.) 



In order to understand the abundance of animal life in 

 various parts and at various depths of the Atlantic, it is very 

 useful to review our knowledge of the distribution of whales in 

 that ocean. I agree with Eschricht in dividing the whales 

 into different biological groups according to the food on which 

 they live. One group feeds on " plankton," another on both 

 plankton and fishes, and a third group on squids. 



Genuine " plankton whales " are the arctic " right " whale 

 (the Greenland whale, Balcena mysticetus, see Fig. 568), and 

 the boreal blue whale i^BalcEnoptera nmsculus, Fig. 569). By 

 the aid of their enormous tongues they press the water out of 

 their mouths between the whalebone lamellae, thus filtering the 

 water and retaining the minute organisms (see Fig. 570). 



