628 THE WONDER OF LIFE 



that it applies, in a measure, to a machine, but a machine 

 is the embodiment of a human purpose. It is an elabor- 

 ated tool, an extended hand, and has inside of it a human 

 thought. 



The Argument from Animal Behaviour. — The 

 inadequacy of a physico-chemical account of vital actiArity 

 becomes even more obvious when we pass from the every- 

 day activities of the body to a connected series of animal 

 activities — to animal behaviour. 



Let us return, for instance, to the newly hatched micro- 

 scopic larva of the liver-fluke, of so much practical import- 

 ance to sheep-farmers (see p. 307). It has no organs in the 

 strict sense ; it has only a few cells altogether ; it has no 

 hint of a nervous system. It is covered with cilia, and 

 has energy enough to swim about for a day in the water- 

 pools by the pasture. It comes in contact with many 

 things, but it responds to none, until haply it touches the 

 little freshwater snail {Lymnceus truncatulus) — ^the only 

 contact that will enable it to continue its life. To this it 

 responds by working its way in at the breathing aperture, 

 and within the snail it goes through a complex series of 

 multipHcations and metamorphoses, the upshot of which 

 may be that a sheep becomes infected with a young liver- 

 fluke. The life-history is dramatic, the risks of failure 

 are enormous ; our point is the delicate adaptation of a 

 brainless organism to the one stimulus which will enable 

 it to continue its life. This seems to us to be far beyond 

 all possibility of mechanical description ; it requires a 

 historical explanation. 



What we have just alluded to is no rare curiosity ; it is 

 a frequent and characteristic feature in animal behaviour 

 that the organism is historically tuned to be a receptor 



