EVOLUTION 237 



grow and if all animal young were to mature, there would soon be 

 no food or room for them on the earth's surface. Only a small 

 fraction can possibly live to maturity. As a result, argues Darwin , 

 there must necessarily be a terrific life-and-death competition, 

 a "struggle for existence", in which the few survive and the many 

 perish. 



Finally, those individuals in this struggle which possess any 

 advantage in structure or in function over their fellows, even 

 if this advantage is a very small one, will evidently have the best 

 chance to succeed and survive. Of the manifold variations which 

 plants and animals display, some will naturally be helpful 

 and some harmful, and those fortunate individuals which vary in 

 the right direction will survive and transmit their advantageous 

 characters to their offspring. The others will perish and leave 

 no descendants. Through this "survival of the fittest" the race 

 tends to change steadily and to progress toward a type which is 

 better and better adapted to the conditions under which it is 

 living, and also to develop new types which can successfully 

 invade new environments. Darwin named this process Natural 

 Selection from analogy to the artificial selection long practiced 

 by man with his domestic animals and plants, by which he has 

 caused such great changes merely by selecting, for breeding 

 stock or for seed production, those individuals which varied in 

 such a way as best to meet his requirements. 



Objections to the Theory. — In support of his theory Darwin 

 brought forth a wealth of evidence so convincing that it won very 

 wide acceptance. As knowledge has advanced, however, various 

 objections to it have persistently been raised. Why is an advan- 

 tageous character appearing in only one individual not lost, by 

 "swamping", in crosses between this individual and the rest of 

 the population? Why do so many structures exist which are not 

 evidently helpful in survival? Why are many species separated 

 by differences so small that it is hard to believe that they are life- 

 and-death differences? What causes the persistence and survival 

 of early steps in the development of a structure, before it has 

 become perfected and useful to the organism ? If variations occur 

 at random, as Darwin supposed, how does it happen that a complex 

 and highly coordinated structure could develop, since its produc- 

 tion would require innumerable variations of just the right degree, 

 in just the right place, and at just the right time? Why have we 



