Bramble-Dwellers 



the whole series from base to top. But those 

 in the lower boxes have a second guide, when 

 the bottom end is open. This is the stimulus 

 of the adjacent air, a more powerful stimulus 

 than that of gravity. The access of the air 

 from without is very slight, because of the 

 partitions; while it can be felt in the nether- 

 most cells, it must decrease rapidly as the 

 storeys ascend. Wherefore the bottom insects, 

 very few in number, obeying the preponderant 

 influence, that of the atmosphere, make for 

 the lower outlet and reverse, if necessary, 

 their original position; those above, on the 

 contrary, who form the great majority, being 

 guided only by gravity, in the case where the 

 upper end is closed, make for that upper end. 

 It goes without saying that, if the upper end 

 be open at the same time as the other, the oc- 

 cupants of the top storeys will have a double 

 incentive to take the ascending path, though 

 this will not prevent the dwellers on the lower 

 floors from obeying, by preference, the call 

 of the adjacent air and adopting the down- 

 ward road. 



I have one means left whereby to judge of 

 the value of my explanation, namely, to ex- 

 periment with tubes open at both ends and 



40 



