GENERAL BIOLOGY 725 



and viscosity occurs. Especially convincing is the fact that 

 although this rise occurs at very different depths in the three 

 localities mentioned, the increase in the volume of small 

 organisms captured in the nets in every case coincides with the 

 rise in the specific gravity. 



An important point for our conception of the animal life of 

 the Atlantic is that the greatest volume of pelagic Crustacea has 

 never been found in the upper 100 or 200 metres, where the 

 production of minute plants takes place ; the great majority of 

 small pelagic Crustacea live everywhere in the deeper inter- 

 mediate layers. The examples cited above show further that 

 the volume of organisms captured differs greatly in correspond- 

 ing depths at the different stations, being strikingly small 

 in the Sargasso Sea com- 

 pared with the boreal waters 

 off Newfoundland and the 

 southern part of the Nor- 

 wegian Sea. 



All these investigations 

 indicate the quantity of 

 organisms present only at 

 the moment of examination. 

 We cannot, from our results, 

 conclude that similar condi- 

 tions always prevail, nor that Fig. 519.— Curves of temperature (O and 

 the a^^reaate quantities of Specific gravity {<r,), station 113. 



r 1 ^ • *i I'll- 1 (North of Wyville Thomson Ridtre.) 



iood-animals which live and 



die during the year are proportionate to the quantities found 

 at a given moment in the different localities. The quantity 

 of food-animals changes first according to seasons and second 

 according to the intensity of production, but very little is 

 known about these two Important factors. Only in restricted 

 areas of the coastal waters have attempts been made to 

 investigate these questions systematically at different seasons, 

 and at present we can only compare the conditions found in 

 different localities. Such comparisons have led us to recognise 

 a vast difference between boreal and subtropical conditions, 

 which we may with advantage consider separately. 



The boreal waters are mainly characterised by great seasonal Seasonal 

 changes. We have previously noted the great seasonal abundance [If^ 

 changes in temperature principally in the surface layers where minute 

 pelagic plants are produced. A no less important part is ""^^^^ea. 

 played by the changes in light intensity from summer to winter 



