122 ORGANISM AND MECHANISM 



trace it through the ordinary metabolic phenomena in living 

 organisms, as well as through the phenomena of senescence, 

 death, and reproduction" (p. 21). 



The everyday life of any common animal is an extraor- 

 dinarily complex affair. " For what is a creature but a 

 great and well-disciplined armj with battalions which we 

 call organs, and brigades which we call systems ? It advances 

 insurgently from day to day, always into new territory of 

 time and space — often inhospitable or actively unfriendly; 

 it holds itself together, it forages, it makes good its ex- 

 penditure of explosives, it even recruits itself, it pitches a 

 camp and strikes it again, it goes into entrenchments and 

 winter-quarters, it retreats and lies low, it recovers itself, it 

 has a forced march, it conquers " (Thomson, Wonder of Life, 

 1914, p. 627). What the biologist wishes is not merely 

 a complete ledger of all the osmotic and capillary proc- 

 esses in the body, all the oxidations and reductions, all the 

 solutions and fermentations, — though that will be a great 

 achievement — he wishes a description of the organism's 

 daily march which Avill not ignore the correlated organismal 

 tactics or the strategy- which, in some cases at least, lies 

 behind these. 



§ 5. Criticism of Mechanistic Descriptions of Animal 



Behaviour. 



Let us pass from the everyday functions of the body 

 to a connected series of external activities — to animal be- 

 haviour, a subject to which we shall return in the sixth 

 lecture. We know that a young British-born swallow which 

 leaves us for the south towards the end of summer may 

 return the following spring to the parish, even to the farm- 

 steading, of its birth, — moved, who shall say by what deep 



