280 



EVOLUTION 



they could not live. A man drowns in 

 the sea; a fish dies out of water. But 

 there are some structures which show 

 a particularly intimate relationship be- 

 tween the organism and its conditions 

 of life. Mistletoe is a parasite that re- 

 quires a tree of certain species to live 

 on, a particular insect to pollinate its 

 flowers, and a thrush to eat its berries 

 and deposit its seeds on branches of 

 the same species of tree. A woodpecker 

 has two of its toes turned backward 

 with which it grips the bark of trees; 

 it has stiff tail feathers with which it 

 props itself against the tree; it has a 

 very stout beak with which it bores 

 holes in the tree trunk; and it has an 

 abnormally long tongue with which 

 it takes the grubs at the bottom of the 

 holes. Other plants than mistletoe and 

 other birds than woodpeckers do not 

 have all these adaptations, and there- 

 fore, if evolution has occurred, it is 

 necessary to give an objective explana- 

 tion of how these adaptations arose. 



Danvin was then able to formulate 

 a complete theory providing a rational 

 explanation of the causes as well as of 

 the fact of evolution in plants and ani- 

 mals. It is formally based on four 

 propositions which he already knew to 

 be true, and three deductions which 

 are now also known to be true. They 

 may be enumerated as follows: 



1. Organisms produce a far greater 

 number of reproductive cells than 

 ever give rise to mature individuals. 



2. The numbers of individuals in 

 species remain more or less con- 

 stant. 



3. Therefore there must be a high rate 

 of mortality. 



4. The individuals in a species are not 

 all identical, but show variation in 

 all characters. 



5. Therefore some variants will suc- 

 ceed better and others less well in the 

 competition for survival, and the 



parents of the next generation will 

 be naturally selected from among 

 those members of the species that 

 show variation in the direction of 

 more effective adaptation to the con- 

 ditions of their environment. 



6. Hereditar)' resemblance between 

 parent and offspring is a fact. 



7. Therefore subsequent generations 

 will by gradual change maintain and 

 improve on the degree of adaptation 

 realized by their parents. 



This is the formal theor}' of evolu- 

 tion by natural selection, first an- 

 nounced jointly on July 1, 1958, by 

 Darwin and Alfred Russel Wallace, 

 who had, again independenth', come 

 to the identical conclusion. It repre- 

 sents a step in knowledge comparable 

 to Newton's discovery of the law of 

 gravitation. 



THE INTEGRATION OF MENDELIAN 

 GENETICS WITH SELECTION 



When Danvin wrote, nothing 

 whatever was known about the laws of 

 heredity, and all that he had to go on 

 was the vague notion that offspring 

 tended to strike an average between the 

 characters of their parents. Tliis sup- 

 position went by the name of "blend- 

 ing inheritance," and it occasioned for 

 Darwin the greatest difficulty with 

 which he had to contend in formulat- 

 ing his theory. In the first place, if 

 blending inheritance were true, it 

 would mean that anv new variation 

 which appeared, even if heritable, 

 would be rapidly diluted by "swamp- 

 ing," and in about 10 generations 

 would have been obliterated. To com- 

 pensate for this it would be necessary 

 to suppose that new variations were 

 extremely frequent. This problem of 

 the supply of variation was a difficulty 

 which Darwin felt so acutelv that it 

 even led him to look for a source of 



