232 HISTORY OF BOTANY 



on the other hand, may be entirely in its favour. Those 

 individuals possessing any variations giving them a 

 superiority, however slight, over their fellows will, on 

 the whole, be more likely to survive than those which 

 have not the variation in question. They will in this 

 way be " naturally selected from among the sum total 

 of individuals of that generation." The useful variation 

 is transmitted to the next generation and so on, until 

 it becomes so marked as to give the organism possessing 

 it the right to claim specific rank. You will observe 

 that the Darwinian theory involves two assumptions, first, 

 that any useful variation is inherited, and, second, that 

 the variation once started goes on increasing and becomes 

 more and more emphasised from generation to generation. 

 But according to the Mendelian hypothesis " 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 the 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 ^y a sudden step, not by a process of 

 gradual and almost imperceptible augmentation. It is 

 not continuous but discontinuous, 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 found, 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. On the old 

 view, no new character could be developed except by the 

 piling up of minute variations through the action of 



