174 MENDELISM chap. 



work they have rapidly crystallised 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 sudden step, not 

 by a process of gradual and almost imperceptible 

 augmentation. It is not continuous, but discon- 

 tinuous, because it is based upon the presence or 

 absence of some definite factor or factors — upon 

 discontinuity in the gametes from which it sprang. 

 Once formed, its continued existence is subject to 

 the arbitrament of natural selection. If of value in 

 the struggle for existence natural selection will 

 decide that those who possess it shall have a better 

 chance of survival and of leaving offspring than 

 those who do not possess it. If it is harmful to 

 the individual natural selection will soon bring about 

 its elimination. But if the new variation is neither 

 harmful nor useful there seems no reason why it 

 should not persist. 



In this way we avoid a difficulty which beset the 

 older view. For on that view no new character 

 could be developed except by the piling up of 

 minute variations through the action of natural 

 selection. Consequently any character found in 

 animals and plants must be supposed to be of 



