376 FOUNDATIONS OF BIOLOGY 



slightly in any manner profitable to itself, under the complex 

 and sometimes varying conditions of life, will have a better 

 chance of surviving, and thus be naturally selected. From 

 the strong principle of inheritance any selected variety will 

 tend to propagate its new and modified form." 



Nothing succeeds like success, and once started Darwin's 

 theory gradually swept all opposition away, and some of its 

 exponents out-Darwined Darwin. Then, as was to be ex- 

 pected, the reaction came. Acquired characters are not 

 heritable; variations are swamped by interbreeding; large 

 variations and not small fluctuating variations are crucial; 

 and so on. But it is not necessary to obscure the main issue 

 by entering, into these controversies. What is the status of 

 the theory of natural selection to-day? 



Evolution is not a closed book an event which has been 

 completed in the past but a process which is actively going 

 on now. "Nothing endures save the flow of energy and the 

 rational order that pervades it." And there is every reason 

 to believe that the factors involved in present evolution are 

 the same as those which have operated in the past. The 

 uniformitarian doctrine has proved productive in explain- 

 ing the evolution of the Earth, and there is every reason to 

 think that this viewpoint will prove is proving equally 

 valuable in understanding the origin of the diverse inhabit- 

 ants of the Earth. We have come to realize that evolution 

 is a bird's-eye view of the results of heredity, since the origin 

 of life the facts of inheritance hold the key to the factors 

 of evolution. Therefore in a previous chapter we discussed 

 the relations of recent discoveries in genetics to the evolution 

 problem, some of which may be restated now with special 

 reference to the origin of the fitness of organisms. 



In the first place we have seen that though variations are 

 the rule and not the exception, some are of importance for 



