The Relativity of Fitness 151 



Fitness Never Perfect 



A large part of the living activities of many species can 

 be carried on only at the expense of other living things. The 

 early bird may be to a certain extent better adapted than the 

 late bird, but if adaptation were equally good on the part of 

 the worms, the bird would never have anything to eat. We 

 cannot have perfect adaptation both for catching prey and 

 for escaping enemies. We cannot have perfect adaptation 

 for a parasitic worm and for its host. If all plant life were 

 protected against destruction by animals, there could be no 

 animal life except such as lived on plants already dead. The 

 very existence of the multitudes of forms points to a pro- 

 gressive adaptation of life in general, to a progressively 

 changing, even progressively more complex, environment. 

 We must constantly bear in mind the fact that the environ- 

 ment of any species includes not only the physical conditions 

 but also other living things. Every change in physical con- 

 ditions entails a change in the amount of plant life and in 

 proportions of the various species of plant. This change in 

 turn brings with it a change in the numbers and propor- 

 tions of animals, which depend upon these various plants for 

 their food. 



Perhaps we can see both the relativity of adaptation 

 and the inevitableness of constant change if we consider 

 what happens in a given region with the advance of the 

 seasons. In the spring as the snow melts away, we see very 

 little of green foliage and practically no insects. A careful 

 search will reveal worms, grubs^, sow bugs and beetles emerg- 

 ing from the thawing soil. The waters and the ponds appear 

 destitute of inhabitants. From day to day, however, green 

 things and moving animals are added to the picture. It is 

 an evidence of adaptation that caterpillars do not put in their 

 appearance until suitable food has already burst from the 

 buds of the trees. It is a part of this same adaptation, in- 

 deed, that the mother butterfly does not begin to lay her 

 eggs until the leaves are already in sight. Still farther back 



