NATURAL SELECTION 397 



Pearl; and of many mutationists who make much greater claims for 

 that theory than does De Vries himself. 



Second. Each generation of biologists is so occupied with its own 

 work and contemporary theories that it makes no real effort to 

 understand preceding theories. 



This second tendency seems to me most marked in the attitude of 

 present workers along genetic lines towards natural selection. They 

 reveal an apparent lack of understanding of what Darwin really meant 

 and of what he claimed; and when criticising that theory they are 

 often engaged in the classic, but unprofitable, exercise of "fighting 

 windmills." 



In view of these facts I hope you will pardon me if I present in as 

 few words as possible just what I believe to be the main factors which 

 Darwin presented as resulting, in their actions and reactions, in 

 natural selection. These factors are three in number: 



First. Heredity, by which the progeny tend to resemble their 

 parents more than they do other individuals of the same species. 



Second. Individual variation, by which the progeny tend to 

 depart from the parental type and sometimes from the specific type. 



Third. Geometrical ratio of increase, by which each species tends 

 to produce more individuals than can survive. 



Each of these factors is practically axiomatic, so little is it open 

 to argument. 



No one doubts the fact of heredity, whether pangenesis, Weis- 

 mannism or Mendelism be the correct expression of the mechanism 

 involved. These do not affect the fact of heredity nor invalidate it 

 as a factor in natural selection. 



No one doubts the fact of variation; whether it is the "individual 

 variation" of Darwin, the "fluctuating variety" or the "mutation" 

 of De Vries. All that is necessary for Darwin's purpose is that there 

 be heritable variations. That there are such things all parties agree 

 and it matters little what you call them. They are adequate to act 

 as a factor in the Darwinian scheme. 



No one doubts the fact of geometrical ratio of increase. It is a 

 proposition easily capable of mathematical demonstration, and that 

 is sufficient for Darwin's purpose. 



These three factors, then, are not debatable as facts, whatever 

 their mechanism or causes. 



A moment's reflection will show that geometrical ratio of increase 

 is a quantitative factor, giving an abundance of individuals from 



