360 



DEPTHS OF THE OCEAN 



Lohmann's 

 pump method 



a very great extent, through the meshes ; and, secondly, the 

 meshes become gradually clogged with the slimy little algae, or 

 animals, so that the coefficient of filtration does not remain con- 

 stant. Even during the course of a single haul we occasionally 

 noticed that everything worked well to begin with, but that the 

 cloth became more and more stopped up, until at last filtration 

 ceased entirely. In other words, it is sometimes impossible to 

 tell how much water has been filtered, and consequently the 

 catch is practically valueless from a quantitative point of view. 



An endeavour was made to overcome this last difficulty by 

 filtering a quantity of water, previously measured, either through 

 silk nets, or through an even less porous filter-material, such as 

 taffeta, or hardened filter-paper, or sand, an additional advantage 

 being that by this means the very smallest organisms could be 

 retained. Water-samples were secured by water-bottles or by 

 pumps. Lohmann, who did much to perfect the pump-method, 

 was not only able to get his water-samples from any depth 

 desired, but could obtain samples representing a column of 

 water from the surface down to a specified level. The pump 

 was made to work in connection with a long, flexible hose, the 

 mouth of which was lowered as far down as considered necessary, 

 and then drawn gradually up towards the surface as pumping 

 proceeded. The pumped-up water thus represented propor- 

 tionally the whole distance through which the hose passed 

 before reaching the surface. These samples were afterwards 

 filtered by Lohmann, and the results compared with catches 

 obtained by vertical hauls with the silk nets. 



The methods of capture had thus been greatly improved, 

 and it was possible to obtain the smallest organisms, but for 

 practical reasons it was necessary to limit the quantity of water 

 filtered on each occasion. This forced us to turn our attention 

 to the second question, namely the regularity with which 

 Distribution of organisms are distributed in the sea. Fortunately, the 

 ?!!!?™„?1^"^^ researches of Hensen and his assistants, as well as those of 

 Lohmann and myself, have all gone to show that the distribu- 

 tion of the pelagic plants, at any rate, is extremely regular. 

 The samples from adjacent localities with similar life-conditions 

 have yielded very concordant results. I do not consider it 

 any exception to this statement that in tropical waters dense 

 masses of Trichodesntium sometimes collect as water- bloom 

 in certain areas and not in others, or that diatoms near 

 the edge of the polar ice occur in more or less local swarms, 

 for I consider it more than probable that these irregularities 



extremely 

 regular 



