RESPONSIVE LIFE OF ORGANISMS 



475 



powder in the mine. In itself it may be insignificant, but in 

 its relations to the organism it is all important; for it deter- 

 mines the very conditions of existence. Nest building must 

 wait on the finding of suitable materials — materials not 

 too different from those the sight of which has elicited the 

 nest building responses in the past, and all the subsequent 

 acts of rearing young wait on nest building. This means 



that the want at any 

 point of a stimulus that 

 can set off an appropri- 

 ate action, blocVs the 

 remaining acts of the 

 series and leads to fail- 

 ure. 



The perfection of 

 instinct at its best is 

 marvellous. Sequences 

 of acts, that, like nest 

 building and web spin- 

 ning, are worthy to be 

 ranked as fine arts, are 

 performed perf e c 1 1 y 

 without instruction, 

 example or experience; 

 performed, perhaps but 

 once in a life time. The 

 method of a caterpillar 

 in spinning is as much 

 a product of its organi- 

 zation as is the silk in 

 its spinning glands. 

 Glands and nerve cells 

 Fig. 266. A caddisfly (Haiesus sp?) whose ^rc aUkc charged during 



sitting posture suggests a half way stage in rlpA^plnTimrrif T Vi ^ -v 



the development of the posture of Molanna. ueveiopnicnL. i n e ) 



