CAUSES OF VARIATION 165 



organisms do vary continuously and in all directions from 

 the characters of the parent organisms, and further than 

 this it is impossible to go with regard to the ultimate cause 

 of variation. Given, however, the smallest variation in the 

 simplest organism, the remainder of the process of evolution 

 is conceivable. Natural selection would increase variability 

 up to the point where it ceased to be useful in adapting 

 a race to its environment, and then would check it. It 

 would appear probable also, that as the modifications pro- 

 duced by the environment in the parent organism would 

 frequently be injurious to the germ cells and to the offspring 

 arising from them, natural selection would eliminate varia- 

 tions in the germ cells that tended to be thus influenced. 

 On the other hand, variations among the germ cells them- 

 selves, as they would tend to give an advantage to the race, 

 would be increased. All other cells vary among themselves, 

 and evidently germ cells do the same. Variations in the 

 germ cells involve variations in the organisms produced 

 from them. There is no evidence that bi-parental reproduc- 

 tion the mixing of the germplasm has any effect in pro- 

 ducing fresh variations. On the contrary, it eliminates 

 useless variations, and is thus a cause, probably the cause 

 of regressive variations. It does not appear to be legitimate 

 to go further than this with regard to the cause of progressive 

 variations. 



