368 DARWINISM AND THE PROBLEMS OF LIFE 



mechanical explanation of the evolution of living 

 hings. 



We know that the theory of selection is a mechanical 

 principle. Just as the pebbles are rolled about in the 

 stream, the smaller ones going farther and the larger 

 remaining behind, so we find the action of natural 

 selection. Chance or causes and effects that are 

 unknown to us, but which we know to be of a 

 mechanical nature determines whether the whole 

 species is to be transformed or whether it divides. 



The principle of selection itself has been generally 

 recognised as mechanical. But this has been denied 

 of its two postulates. It has been said that the varia- 

 tions are not universal, or do not diverge in all directions, 

 but only in a few. If the variations were universal why 

 could we not produce, for instance, a cock's spur on a 

 pigeon by selection ? 



We need not delay with this difficulty, as we settled it 

 in dealing with the mutation-theory in the previous 

 chapter. We cannot breed a spur on the pigeon 

 because we do not know what varieties of pigeons to 

 select for the purpose. That the occurrence of varia- 

 tions is not the same in every species is clear from the 

 fact that many animals of very different classes have 

 assumed the same form. I need only mention the 

 parasitism of the spiders, crabs, and worms. 1 



1 If we wish to form a scientific and harmonious conception of the 

 world, we can only use mechanical principles in explaining it. 

 Mechanical natural selection can do nothing without postulating 

 variations. Hence if we wish to explain the organic world mechani- 



