CH. XI.] TWO OBJECTIONS CONSIDERED, 47 



6. But the environment of every group of organisms is 

 steadily, though slowly, changing ; 



7. Every group of organisms must accordingly change in 

 average character, under penalty of extinction ; 



8. Changes due to individual variation are complicated by 

 the law that a change set up in any one part of a highly 

 complex and coherent aggregate, like an organiara, initiates 

 changes in other parts ; 



9. They are further complicated by the law that structures 

 are nourished in proportion to their use ; 



10. From the foregoing nine propositions, each one of 

 which is indisputably true, it is an inevitable corollary that 

 changes thus set up and complicated must eventually alter 

 the specific character of any given group of organisms ; 



11. It is postulated that, since the first appearance of life 

 upon the earth's surface, sufficient time has elapsed to have 

 enabled such causes as the foregoing to produce all the 

 specific heterogeneity now witnessed. 



It seems to me that this summary fairly represents the 

 logical character of the theory of natural selection. The 

 theory is so strong that no scientific writer is disposed to 

 deny that the process of natural selection has always gone 

 on and must continue to go on. And the inference cannot 

 be avoided that in due course of time the process must work 

 specific variations. The only purely hypothetical portion of 

 the theory is the assumption that past geologic time has been 

 long enough to allow of the total process of evolution by 

 such infinitesimal increments. But concerning this assump- 

 tion, it is the clear verdict of logic, that if the theory is 

 thoroughly substantiated in all its other portions, we have 

 the right to claim as much time as is needful, provided we 

 do not run counter to conclusions legitimately reached by 

 astronomy, geology, or physics. Now concerning the age of 

 the earth, neither astronomy, nor geology, nor physics, has as 

 yet had anything conclusive to say ; and it must be left for 



