jj'ebruakt 11, 1921] 



SCIENCE 



129 



'70's was from three to five mothers in every 

 hundred, and sometimes childbed fever raged 

 in epidemic form and killed at the rate of 20, 

 40 and even 55 mothers in every hundred! 

 , Novf, this most beautiful of all human rela- 

 tions has been made safe — ^mark my words — 

 made safe by the researches, especially of Pas- 

 teur and his successors. Bacteriology has won 

 this splendid victory. "Within the last decade, 

 series of 6,000, 7,000 and even over 8,000 eases 

 have been reported without the death of a 

 single mother from infection. Is not that a 

 cause for a Te Deum ? 



But I must call a halt though I have not 

 told even a small fraction of the fascinating 

 Story, of what, remember, I have been an en- 

 thusiastic living witness. 



And what of the future? Have we any rea- 

 son to expect other astonishing and beneficent 

 discoveries? I answer with an unqualified 

 affirmative. And it may well be still greater 

 and still more beneficent discoveries. 



"With this word of cheer, I face the coming 

 year or, if it so please God, the coming years, 

 with a confidence which is enhanced by your 

 wonderful tribute of affection. 



THE RELATION OF MENDELISM AND 



THE MUTATION THEORY TO 



NATURAL SELECTIONi 



Two marked tendencies are evident in the 

 history of any important theory after its pub- 

 lication. 



First. The followers of the discoverer carry 

 the theory too far and attempt too universal 

 an application. This is manifestly true of 

 Wallace and Weismann who out-Darwined 

 Darwin in their claims for natural selection; 

 of the followers of Mendel, such as Morgan 

 and Pearl; and of many mutationists who 

 make much greater claims for that theoi-y 

 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 under- 

 stand preceding theories. 



1 Bead before the American Society of Natural- 

 ists at Chicago, Deeemtier 31, 1920. 



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 imderstand- 

 ing- of what Darwin really meant and of what 

 he claimed; and when criticising that theory 

 they are often engaged in the classic, but un- 

 profitable, exercise of "fighting windmills," 

 In view of these facts I hope you will par- 

 don me if I present in as few words as pos- 

 sible just what I believe to be the main fac- 

 tors which Darwin presented as resulting, in 

 their actions and reactions, in natural selec- 

 tion. 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 reproduce more 

 individuals than can survive. 

 Each of these factors is practically axio- 

 matic, so little is it open to argument. 



ISTo one doubts the fact of heredity, whether 

 pangenesis, "Weismannism or Mendelism be 

 the correct expression of the mechanism in- 

 volved. These do not affect the fact of 

 heredity nor invalidate it as a factor in nat- 

 ural selection. 



N'o 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 nec- 

 essary 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 it is 

 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 geo- 

 metrical ratio of increase is a quantitative 

 factor, giving an abundance of individuals 



