ARTIFICIAL AND NATURAL SELECTION 37 



must lead to structural modifications ; and that this 

 is the clue to the unlikeness between the fauna of 

 successive geologic ages. 



I propose first to give an outline of the argu- 

 ment and to consider the arrangement given in Mr. 

 Wallace's chart of the Theory of Natural Selection. 



P 



WALLACE'S CHART OF THE THEORY OF 

 NATURAL SELECTION. 



PROVED FACTS. CONSEQUENCES. 



A. RAPID INCREASE OF NUMBERS— STRUGGLE 



B. TOTAL NUMBERS STATIONARY— I FOR EXISTENCE 



C. STRUGGLE FOR EXISTENCE — SURVIVAL 



D. VARIATION WITH HEREDITY —I OF THE FITTEST 



E. SURVIVAL OF THE FITTEST — ,_ STRUCTURAL 



F. CHANGE OF ENVIRONMENT —I MODIFICATIONS. 



/ a. All animals produce far more young than can 

 \urvive. Consider for instance the enormous number 

 of eggs of fish or oysters. 



b. The total numbers are on the average stationary. 

 As a necessary consequence of this there is a struggle 

 for existence, because there is neither enough space 

 nor food for all. 



d. Variation with heredity. No two animals are 

 exactly alike, and their distinctive characters are 

 transmitted from generation to generation. The 

 consequence of this is the survival of 'the fittest — that 

 is, that in the long run those best adapted to their 

 circumstances and environment will have the best 

 chance of surviving, and of leaving descendants who 



