304 G. p. BIDDER 



section of the tube, so that the water entering has this trans- 

 verse area of channel as it passes the flagella (Text-fig. 8), and 

 therefore the velocity of only 6 ^x a second.-^ Below the flagella 

 half the channel is occupied by the necks of the collars, so that 

 between them the water moves at the rate of 12/;>t a second, 

 and a particle of food takes a second to travel the length of a cell. 



Slow as we thought the movement of the flagella, at | mm. 

 a second, the water on which they act is stationary by 

 comparison, and they can get on their full work. And the 

 remarkable anomaly in sponges, that through considerable 

 evolution their motor-cells remain their ingestive cells, ceases 

 to be surprising when we realize that both functions are alike 

 localized at the position where the current is slowest. 



The one second during which a food-particle is passing the 

 collar-cell is not such a short time for its capture as w^ould at 

 first appear. It allows of a good many events in the cell's life : 

 we know of twenty double vibrations of the flagellum, with the 

 metabolic cycles which they imply. The biological magnitude 

 of an interval of time is measured by the number of events 

 which can happen in it ; and since every event requires the 

 motion of something from one position to another, therefore 

 where the distances between positions are smaller, events can 

 happen more rapidly. Every motion is produced by an 

 acceleration — such as gravity, or the stress of contracting 

 protoplasm — and with a given acceleration the time required 

 to move over a certain distance from rest is as the square root 

 of that distance : a stone takes a second to fall 16 ft., but to 

 fall a quarter that distance takes half a second. Therefore in 

 a biological world whose linear dimensions are toVo those 

 of our own, there will be some thirty times as many events 

 in a second as in our own, since thirty-two times thirty-two 

 is a thousand ; and I suggest that in the biological time of the 

 flagellate cell the one second during which a particle of food is 

 passing would compare with half a minute in our external life. 

 (We must put the adjective ' external ', because our psychical 

 events which happen ' with the speed of thought ', are events 

 ^ Note 6 gives reason for supposing double these velocities in full health. 



