\ 



\ 



62 THEORETICAL BIOLOGY 



of our knowledge of the world we live in, if the same number 

 of moment-signs had to cope with the content of from 800 to 

 8000 solar years. 



These speculations of von Baer's teach us that, assuming 

 sensory activity to remain unchanged, our world of movement 

 would come to a standstill, if its duration were abbreviated or 

 lengthened beyond a certain point. 



Let us take as an example the spoke of a wheel, the turning 

 of which we can clearly recognise, and let us first slow down 

 the motion, and then speed it up. The movement is of the 

 same kind throughout, but in the first case we accompany it 

 with many moment-signs and in the second with few ; never- 

 theless, in both the movement ceases. If the spoke goes 

 round as slowly as the minute-hand of a watch, it seems to 

 stand stiU all the time, because the change in its position is so 

 slight that we do not perceive it. If we let the spoke whirl 

 round very quickly, we see nothing in the wheel but a uniform 

 clear area, which, as the revolutions continue, persists as a 

 sort of thin veil. 



The same thing would result if we were to crowd the 

 world phenomena into very short duration, or if we were to 

 stretch them out very greatly, so that in the first case they 

 were framed, as it were, by too many moment-signs, and in the 

 other by too few. If there are too many frames, the pictures 

 that succeed one another are too much alike ; if there are too 

 few, the content of many pictures is squeezed into one. In 

 the first case, a ball thrown into the air would stand still ; 

 in the second, the sun would describe a gleaming curve across 

 the sky. 



The rule runs as follows : — a moment ceases to be per- 

 ceived if its gradient is either too steep or too level. 



