GENERAL BIOLOGY ^^^ 



importance, but to determine the exact quantity of something 

 clearly defined, as, for instance, the number of individuals of 

 certain definite species living in a sharply limited water-layer, 

 is of the highest interest. 



When planning the Atlantic cruise of the " Michael Sars " I 

 considered it our first duty to investigate in a qualitative way 

 what organisms live at the various depths. For this purpose 

 we made many determinations of quantity (see Chapter IX.), 

 for instance, in order to illustrate the abundance of certain 

 species in each of the appliances towed at different depths. 

 This method made no pretence of giving absolute figures, but 

 it gave us certain ideas regarding the relative quantities of 

 organisms living at different depths, and the figures obtained 

 by counting the fishes in our trawlings are of a similar kind. 

 My opinion is that these estimates represent the natural 

 conditions better than the ideas regarding animal life in the 

 Atlantic gained by the German Plankton Expedition ; this 

 ocean, being inhabited by organisms at all depths, is very far 

 from being as poor as shown by the nettings of the Plankton 

 Expedition. At the surface reproduction must be exceedingly 

 rapid, or else it would be perfectly inconceivable that so 

 many animals could live in the deeper water, unless, indeed, 

 their nourishment were derived from distant localities, a 

 question that future investigations must answer. Further, the 

 peculiar difference between the quantities of organisms 

 occurring in the deep water of boreal and of warm oceanic 

 waters will have to be more closely studied. In the ocean we 

 find first a minimum just below the surface, then a pronounced 

 maximum, with probably a minimum again in the deeper waters 

 (see Chapter IX. on capture of Cyclothone in closing-nets at 

 Station 63). I suggest as a working hypothesis that this is 

 due to the peculiar distribution of specific gravity and viscosity, 

 which is quite different in boreal and in warm oceanic waters. 



When speaking of floating, I mentioned how the distribution 

 of temperature, and consequently of specific gravity and viscosity, 

 affected the geographical distribution of species, and in Chapter 

 IX. I drew a limit between boreal and warm - water forms, 

 which on the whole, horizontally and vertically, coincided with 

 the isotherm of 10° C. In thus employing temperature alone 

 as a means of dividing animal-communities my idea has only 

 been to consider the temperature as an indicator of certain 

 climatic conditions on which animal life is dependent. From 

 this point of view let us inspect a section of the Atlantic along 



