Looking Baokwa/rd 



to frankly admit that what are styled causes 

 are really only carefully framed theories which 

 seem to account for observed facts. But to 

 sum up in a very few words, it may be said 

 that there seems to be an inborn tendency in 

 living things, both plants and animals, to vary 

 and to adapt themselves to circumstances. 

 Changes in their surroundings and these are 

 ever taking place simply allow this natural 

 tendency a chance to act. The simpler the 

 creature and the more uniform the surround- 

 ings, the less would be the tendency to vary. 

 The more complex the structure of an animal 

 and the more variable the conditions to which 

 it was subjected, the more liable would it be 

 to undergo some change. And as more and 

 more highly organized animals appeared on the 

 scene the more rapidly would changes take 

 place. 



Thus, some of the simple animals that dwell 

 in the depths of the sea, where quiet, darkness, 

 and cold prevail, have a history that reaches 

 back into the past for lengths of time almost 

 inconceivable to us, amounting to millions of 



275 



