THE WONDER OF LIFE 629 



to particular but absolutely indispensable stimuli which 

 may not occur more than once in the life-history. The 

 freshwater mussel, as we have already mentioned, carries 

 her young ones in her outer gill-plate, and does not set 

 them free unless a stickleback or a minnow or some other 

 such fish is in the immediate vicinity. When the fish comes 

 near, the mother mussel, whom it is no libel to call ' acepha- 

 lous ', liberates a crowd of pinhead-like larval mussels or 

 Glochidia, who rush out into the water like boys from the 

 opened school door. They snap their minute valves ; they 

 make for the fish ; they fasten on its skin and enter upon 

 a new chapter of their life-history. Even in the laboratory, 

 when they have been removed from the mother, they be- 

 come excitedly active if a morsel of stickleback be dropped 

 into the dish in which they are. It is this organic memory 

 of the essential stimulus that seems to us to be character- 

 istic and supra-mechanical of a higher order than the 

 responsiveness of wires or photographic plates to particular 

 kinds of rays. It is a sensitiveness gained or invented by 

 the creature in the course of its racial evolution and regis- 

 tered in the constitution. Though simpler, it is as well 

 marked in the absolutely brainless larva of the liver-fluke 

 as in the larval mussel which has the beginnings of a nervous 

 system; in a small-brained, predominantly instinctive 

 creature like a bee as much as in a big-brained, predomin- 

 ably intelligent creature like a bird. We find analogous 

 kinds of behaviour at all levels of nervous organization. 

 The worker-bee leaving the hive for the first time enters 

 a new world with confidence and proceeds to gather 

 honey from difficult flowers, being ' to the manner born '. 

 We have referred to the definite proof that a young 

 swallow which leaves Britain for the South at the end of 



