228 THE WONDER OF LIFE 



Similarly, when Fabre wickedly joined the front end of a 

 file of procession caterpillars to the hind end, they went 

 on circling round and round the stone curb of a big vase 

 in the garden, day after day for a week, covering persist- 

 ently many futile metres. As Fabre said : ' Ils ne savent 

 rien de rien '. 



Alfred G. Mayer and Caroline G. Soule made some inter- 

 esting experiments on the caterpillars of the milk-weed 

 butterfly (Danais plexippus). Thus they observed that 

 once the caterpillars have started eating, they may be 

 induced to eat substances which they would never have 

 begun with. Although they are not receiving the proper 

 stimulus, they cannot stop. This tendency to continue 

 activity * in the face of a non-stimulus ' is called l the 

 momentum of the reaction '. Another interesting point is 

 the shortness of their associative memory. If a ' distasteful ' 

 leaf is presented at intervals of one and a half minutes, the 

 caterpillar tries it every time and takes about the same 

 number of tentative bites. But if the leaf be presented at 

 intervals of about thirty seconds, the caterpillar takes fewer 

 and fewer bites, and then refuses. But it cannot remember 

 for a minute and a half. 



The limitations of instincts are very interesting, especially 

 in showing how different instinctive behaviour is from 

 intelligent behaviour, but it must be emphasized that it is 

 part of the conception of an instinct that it shall be service- 

 able from the start. To a greater or less extent it must be 

 serviceable for survival in the widest sense, and serviceable 

 also * as affording the congenital foundations for an im- 

 proved superstructure of behaviour '. Even though it is far 

 from perfect, even though it is afterwards greatly improved, 

 even though it is only a play instinct (which is far 



