358 GENERAL ZOOLOGY 



somes offers a satisfactory solution of the traditional problem 

 of heredity." 



There is perhaps some question as to whether hereditary 

 characters may be modified by selection. That is, whether 

 selection in a given direction may cause variation in the 

 same direction. The bulk of the evidence, however, ap- 

 pears to indicate that such is not the case. According to 

 Morgan, then: 



" Evolution has taken place by the incorporation into the 

 race of those mutations that are beneficial to the life and 

 reproduction of the organism. Natural selection as here 

 defined means both the increase in the number of individuals 

 that results after a beneficial mulation has occurred (owing 

 to the ability of living matter to propagate) and also that 

 this preponderance of certain kinds of individuals in a 

 population makes some further results more probable than 

 others. More than this natural selection cannot mean, if 

 factors are fixed and not changed by selection." 



The question of evolution must be left in this state: the 

 fact that there has been an evolution is proved, the methods 

 by which evolution takes place have during the past few 

 years become pretty well known, but the causes of variation 

 and heredity are as yet largely unknown. 



