536 BIOLOGY AND HUMAN LIFE 



when pursued by a larger one, or the one that can catch his prey 

 quickest when there are smaller ones around, or the one that is 

 able to make use of a great variety of food. There is no 

 absolute standard for plants or animals. Fitness is a relation 

 between the organism and all of its surroundings, including 

 possible enemies and possible food, changes in physical condi- 

 tions and in seasons. The wolf and the vulture are just as fit 

 as the sheep and the chicken. The thistle and the ragweed are 

 just as fit as the sunflower and the pansy. But no plant species 

 and no animal species can altogether fit in where some other one 

 is living. The fitness is of a special kind that has taken hun- 

 dreds of thousands of years to attain. When conditions in any 

 region change radically, we can see that the whole character of 

 the vegetation changes, and also the character of the animal life. 

 387. A weeding-out process. Only a small fraction of each 

 new generation can survive. The majority is always destroyed. 

 Which particular ones are destroyed is determined by a multi- 

 tude of details— details of structure, details of response, details 

 of activity. All the details together bring about a degree of 

 fitness to the total surroundings— or lack of fitness. One ani- 

 mal was killed early because it was too slow, another because it 

 moved into the open too soon, before its enemy was out of sight. 

 One animal was killed because the tapeworm in its intestines 

 was too much for it ; another because it was too conspicuous 

 on a tree, and a bird caught it. On the whole the total quali- 

 ties of the surviving members of a species are probably better 

 suited to the particular conditions of life than the total quali- 

 ties of those members who perished. Charles Darwin gave the 

 name natural selection to this process whereby the least adapt- 

 able members of a species are weeded out, leaving the most 

 adaptable ones to survive and reproduce themselves. Can we 

 imagine that the weakest or the slowest or the most easily wilted 

 or winded individuals would survive? Of course not. So far the 

 expression natural selection is simply a shorthand phrase for 

 describing exactly what our common sense would lead us to 

 expect ; and Darwin used this term not to imply that nature 



