376 CLASS xvi. 



that half of the earth in which winter prevails ; and whilst thus in 

 one half of the year the number of living creatures from the north 

 pole to the equator, and from the equator to the south pole in- 

 creases, the stream turns northward again in the other half of the 

 year when the southern hemisphere feels the benumbing power of 

 winter. Here too we should consider the peculiar structure of 

 birds, by which, being brought into closer relations with the 

 atmosphere, they are enabled to feel its influence more than other 

 animals, and so are in a better condition to discover changes in it 

 by which they may direct their acts. To maintain that birds 

 migrate from deficiency of food is, as it seems to me, to substitute a 

 final cause for an active cause, or in other words to confound the 

 wherefore with the whereby. 



