776 DEPTHS OF THE OCEAN chap. 



these animals are absent, and the volume is obviously at a 

 minimum. 



We may therefore assert that the small nets actually capture 

 a purely accidental selection of the animals present, and that 

 the use of the nets only above 200 metres gives a merely 

 casual selection, which is by no means a characteristic gauge as 

 to the quantity of organisms living beneath a square metre of 

 surface even at the moment. 



Is the idea of a certain quantity per square metre of 

 surface on the whole of any value whatever as regards the 

 ocean ? We may speak about the quantities produced per 

 hectare or per square metre of soil, and we may also classify 

 the production of a pond ; but is there in the ocean any connection 

 whatever between the different layers of a column of water 5000 

 or 6000 metres deep by i metre square in regard to the 

 vertical exchange of nutritive substances? Is it not probable 

 that this exchange takes place in an oblique direction and at 

 various angles at different depths ? At the surface of the North 

 Atlantic the Gulf Stream in many places runs with great 

 velocity, but how deep this current extends, or, to put it more 

 correctly, at what depths it runs in the same direction and 

 with the same velocity, is indeed as yet almost unknown. 

 Below this current there are perhaps in places powerful 

 reaction currents, running in opposite or other directions, 

 probably with a considerable vertical range (see current 

 measurements described in Chapter V.), and these would 

 have to be passed through before reaching depths where the 

 water layers move very slowly or not at all. Bodies sinking 

 from the productive plant-stratum at the surface must, therefore, 

 be supposed to be carried far away in a horizontal direction 

 before reaching deep water. The nutriment of the deep layers 

 of any locality is thus not derived from a point situated exactly 

 above it, but has probably come from some very distant point, 

 and the fact that boreal forms are found in deep water below 

 the warm waters of the south may be a corroborative proof of 

 this. 



Notwithstanding my admiration for Hensen's methods, I 

 have always held that before these methods can be applied in 

 nature we must make a qualitative investigation, to be followed 

 by an investigation as to the relative quantities of the organisms 

 present, in order to define the selection which must be made if 

 we wish to determine the absolute quantities. To define the 

 quantity of something perfectly casual is indeed of little 



