THE INSURGENCE OF LIFE 177 



humidity, and food supply are such as to exclude, for 

 many kinds of birds, the possibility of nesting in the south. 

 Perhaps in some cases the bird's constitution is such that it 

 cannot become reproductive without the subtle stimulus 

 implied in a return to the conditions of the original birth- 

 place. Perhaps too there are lingering memories of the 

 abundant and pleasant food whether berries or mos- 

 quitoes to be had in the North. Both on the repro- 

 ductive and on the nutritive side there may be a sort of 

 constitutional home-sickness. 



It is difficult to get beyond mere speculation in regard 

 to the origin of the migratory activity. The living organ- 

 ism is not merely a responsive plastic system which the 

 environment subjects to various experiences ; it is a crea- 

 ture that experiments. Migration was an experiment, an 

 ' inborn inspiration ', probably to begin with of germinal 

 origin in the face of untoward conditions. The new line 

 of solution, peculiarly natural to a flying creature, was to 

 evade the difficulties, instead of facing them. Thus, instead 

 of hibernating or laying on fat or making a great store of 

 food, birds migrated before the approach of winter. It 

 was a stroke of genius to discover that the prison doors 

 were open. 



Our view, then, is this, that an original instinctive 

 mutation must be postulated, which amounted to ' a new 

 idea ', but was not an idea, which found expression in a 

 timeous restlessness, in sensory alertness, in adventurous 

 experiment, and in a power of flying more or less in one 

 direction. Perhaps we see something like the beginning of 

 it to-day in animals which seem to be sensitive to remote 

 warnings of an impending storm, and take refuge accord- 

 ingly. Given a beginning, we can understand the diffusion, 



