xm VARIATION AND EVOLUTION 141 



tinction. Were it possible in such a case to have all the 

 forms before us, they would present the appearance of a 

 long series imperceptibly grading from one extreme to the 

 other. 



Upon this view are made two assumptions not unnatu- 

 ral in the absence of any exact knowledge of the nature of 

 heredity and variation. It was assumed, in the first place 

 that variation was a continuous process, and, second, that 

 any variation could be transmitted to the offspring. 

 Both of these assumptions have since been shown to be 

 unjustified. Even before Mendel's work became known 

 Bateson had begun to call attention to the prevalence of 

 discontinuity in variation, and a few years later this was 

 emphasised by the Dutch botanist Hugo de Vries in his 

 great work on The Mutation Theory. The ferment of 

 new ideas was already working in the solution, and under 

 the stimulus of Mendel's work they have rapidly crys- 

 tallised out. With the advent of heredity as a definite 

 science we have been led to revise our views as to the 

 nature of variation, and consequently in some respects as 

 to the trend of evolution. Heritable variation has a 

 definite basis in the gamete, and it is to the gamete, 

 therefore, not to the individual, that we must look for 

 the initiation of this process. Somewhere or other in the 

 course of their production is added or removed the factor 

 upon whose removal or addition the new variation owes 

 its existence. The new variation springs into being by a 



