154 THE EVOLUTION OF MAN 



any human being can predict particular events that depend 

 upon so many factors as are involved in the development of 

 an individual or the evolution of a race, that I venture to 

 direct your attention to the trend of human evolution. 



CAUSES AND DIRECTIONS OF PAST EVOLUTION 



And first it is necessary to consider the chief causes of evo- 

 lution in the past and the most important paths which it has 

 followed, for there is no way of looking into the future except 

 by the light of the past. Unfortunately our knowledge of 

 the causes of evolution is not very complete, but the majority 

 of biologists agree that inherited variations, or mutations, 

 constitute the building materials of evolution, while natural 

 selection, or the elimination of the unfit, is the workman or 

 architect that selects or rejects these materials. 



Inheritance and Variation. In order to be of any evolution- 

 ary value a variation must be inherited. Thousands of varia- 

 tions occur in organisms which are not inherited; they come 

 with changes in food, climate, use or disuse or other conditions 

 of environment and when these conditions change they disap- 

 pear. These environmental variations are known as "fluctua- 

 tions" ; they represent changes in development rather than in 

 heredity, modifications of the developed organism rather than 

 of the germ-plasm. On the other hand, inherited variations 

 are caused by changes in the germ-plasm itself. These changes 

 may be of two kinds, ( i ) those which are due to new com- 

 binations or recombinations of old inheritance factors, or what 

 is known as "Mendelism," and (2) those which are caused by 

 sudden alterations in the individual factors or genes, such 

 transformations being known as "mutations." 



Formerly these three types of variation, namely, fluctuations, 

 Mendelian combinations, and mutations, were not clearly dis- 

 tinguished, and Darwin assumed that all kinds of variation 



