DESCENT WITH CHANGE 377 



tend to survive and to pass on the favorable variation to its prog- 

 eny. Thus by natural selection is brought about the survival 

 of the fittest — the survival of those individuals, and there- 

 fore species, which are best adapted to the peculiar conditions 

 of their environment and mode of life. And note, this offers an 

 explanation of the fact of adaptation itself — perhaps the most 

 striking phenomenon which organisms exhibit. 



This is all so simple from one point of view and so confusingly 

 complex from others that it may well be restated in a couple of 

 sentences by Darwin himself: "As many more individuals of each 

 species are born than can possibly survive, and as, consequently, 

 there is frequently recurring struggle for existence, it follows that 

 any being, if it vary however 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 nearly all opposition away. Indeed, some of its 

 advocates in their enthusiasm extended Darwin's theory to a 

 point not justified by his own conservative statements. Then, 

 as was to be expected, the reaction came. One objection after 

 another was raised as the problem was studied from nearly every 

 standpoint by biologists the world over. But it is unnecessary to 

 obscure the main issue by entering into these controversies. What 

 is the status of the theory of natural selection to-day? The answer 

 must be sought in the light of genetics. (Figs. 180, 239.) 



3. Genetics and Evolution 



Evolution is not a closed book — an event which has been com- 

 pleted in the past — but a process which is actively going on now. 

 It may well be an accelerating process that is gaining momentum. 

 Perhaps it is even to-day but little beyond the beginning of its 

 revelations. 'Nothing endures save the flow of energy and the ra- 

 tional order that pervades it." And there is every reason to be- 

 lieve that the factors involved in present evolution are the same 

 as those which have operated in the past. This uniformitarian 

 doctrine has proved productive in explaining the evolution of the 

 Earth, and all the available evidence indicates that this view- 

 point will prove — is proving — equally valuable in understand- 



