362 



DEPTHS OF THE OCEAN 



determining the species whose development we desire to trace ; 

 consequently most of those who endeavour to work at these 

 interesting questions will be forced to confine themselves to 

 definite problems, and content themselves with tracing the 

 growth of a limited number of species. No doubt a man like 

 Lohmann may be able to know all the species within certain 

 limits, and may actually calculate by counting what each of 

 them contributes to the total plankton volume, but speaking 

 generally a " uni- 

 versal method " that 

 will give us the total 

 quantity of all the 

 plants and animals 

 of the sea in curves 

 and tables is un- 

 attainable. 



During the 

 " Michael Sars " 

 Expedition our 

 quantitative investi- 

 gations yielded really 

 remarkable results. 

 Lohmann had suc- 

 ceeded by means of 

 a centrifuge in de- 

 termining the quan- 

 tity of plankton in 

 quite small samples 

 of Baltic water, and 

 we felt confident, 

 therefore, that this 

 excellent method 

 ought also to prove 

 serviceable in the 

 open sea. We very soon found, however, that the algae there 

 were too scarce for our little hand-centrifuge (Fig. 250) to be 

 of much utility ; there was so little to be found at the bottom 

 of the centrifuge glasses (Fig. 251) that we obtained a hope- 

 lessly inadequate idea of the plant life, whereas in the 

 stomachs of salpse we might, perhaps, get a quite abundant 

 flora of small forms. Fortunately, we had taken with us a 

 big centrifuge to be worked by steam (see Fig. 91, p. 105), 

 and in its six glasses we could centrifuge at one time as much 



Fig. 250. — Lohmann's Hand-centrifuge. 



