68 VIGNETTES FROM NATURE. 



darker, larger or smaller, longer in this limb 

 and shorter in that, than one another it 

 must follow that each individual must be 

 slightly better or worse adapted for surviving 

 in certain special circumstances than some 

 other. Such petty differences are for ever 

 being produced, and the better are constantly, 

 on the average, living down the worse, while 

 the worse are being constantly weeded out. 

 If at any moment we look at the world as a 

 whole, we see apparent balance, nay rather 

 real temporary balance ; every part fits in to 

 every other with absolute adaptation. But if 

 we compare widely different times we see 

 that the balance is always altering, that types 

 change indefinitely from age to age. Each 

 plant and each animal fulfils all the necessary 

 conditions of existence every day of its life, 

 or else ceases to exist, the survivors being 

 always those who have succeeded in fulfilling 

 them ; but then the conditions are always 

 slightly changing, and so the survivors from 

 time to time are slightly different. Every 



