118 EVOLUTION AND GENETICS 



interpretation. Darwin by no means restricted the 

 application of the term natural selection to the sub- 

 stitution for the parent type of a better adapted new 

 type. Perhaps it is owing to the various ways in 

 which he used natiu*al selection — often as a meta- 

 phor — that it has come to have so many different 

 meanings and is often confusedly used as synony- 

 mous with evolution. 



Progressive Evolution 



It has been pointed out that the power to reproduce 

 itself puts the problem of the construction of a liv- 

 ing organism on a different footing from the con- 

 struction of a complex machine out of inorganic (not 

 living) material. This question is so important for 

 the theory of evolution that its significance must 

 be fiu'ther indicated. 



Whenever a variation in a new direction becomes 

 established the chance of further advance in the 

 same direction is increased. An increase in the num- 

 ber of individuals possessing a particular character 

 has an influence on the f utiu'e course of evolution, — 

 not because the new type is more likely to mutate 

 again in the same direction, but because a mutation 

 in the same direction has a better chance of produc- 

 ing a further advance since all individuals are now 

 on a higher level than before. When, for example, 

 elephants had trunks less than a foot long {fig. 63) 

 the chance of getting trunks more than one foot 



