FITNESSES 



in upon one another, the spikes on one side fitting 

 in between those on the other, like the teeth of a rat 

 trap. This happens quickly, but the movement 

 stops before the leaf is completely closed a fact 

 which Darwin explained ingeniously. Insects of 

 small size, hardly worth catching, escape between 

 the crossed teeth, and the leaf soon reopens. A 

 larger victim, unable to get through the prison bars, 

 touches the sensitive hairs again and induces a 

 second and more vigorous contraction, which proves 

 fatal. No more effective adaptation could be 

 imagined. 



Let us follow the story a step further. When the 

 Venus Fly Trap is tricked into closing, it opens again 

 in twenty-four hours. But when it shuts on a big 

 fly it remains with the two lobes against one another 

 for a week or more. The secreting glands are 

 stimulated and pour out digestive juice ; more and 

 more glands join in ; and the two blades bulge 

 outwards partly with the fly and partly with the 

 copious digestive juice. It is interesting to notice, 

 as an instance of the kinship of all living creatures, 

 that the closing of the Fly Trap leaf is accompanied 

 by an electrical change similar to that associated 

 with every muscular contraction. 



Snow Shoes 



One might spend a pleasant lifetime in admiring 

 organic adaptations, and even the most matter- 

 of-fact man must admit that many of them are fine 

 examples of attaining effective results by very 



61 



