THE DRIFTING SWARMS 



they float near the open sea where the rays of the sun can 

 penetrate to them. Owing to their microscopic size they 

 have a high ratio of surface in relation to volume, and 

 therefore sink very slowly although they are slightly 

 heavier than water. Small organisms, other things being 

 equal, sink more slowly than large ones. The same is true 

 of creatures which fall through air. As Professor Haldane 

 expresses it in his Possible Worlds : "You can drop a mouse 

 down a thousand-yard mine shaft, and on arriving at the 

 bottom it gets a slight shock and walks away. A rat 

 would probably be killed, though it can fall safely from 

 the eleventh storey of a building." Diatoms which have 

 spines — thereby increasing their surfaces in relation to 

 their volumes — can fall even more slowly, just as the 

 puff-balls of dandelions are able to float in the air 

 because their surface areas are so greatly disproportionate 

 to their volumes. Lead pellets of the same volume as the 

 puff-balls would fall quickly. 



This is the basic principle which controls the level of 

 the diatoms in the sea, and affects their availability as 

 food for all kinds of sea animals. 



Diatoms normally reproduce by simply dividing in 

 two, and this act of reproduction naturally affects their 

 distance from the sea's surface. Instead of one box there 

 are now two boxes, each containing a nucleus of living 

 matter — protoplasm — out of which develops the new 

 valves, or halves of the new box. The process of separa- 

 tion is baffling and inexplicable — as mysterious as the 

 division of cells which results in the creation of the 

 foetus within the womb of a woman. But there is this 

 vital difference in the two kinds of fission. Human cells 

 multiply as the infant is built up, but do not diminish in 

 size. But as the diatom "boxes" multiply, some of them 

 get smaller. The process of repeated division by forming 

 new half-boxes within the old ones necessarily causes 

 this diminution in size. Thus there is a wide range 

 in the size of diatoms of the same species, although 



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