24 



not been positively demonstrated— the 

 statements of biometricians apply, as 

 has been frequently pointed out, to 

 populations which are not but could 

 be divided into pure lines. The burden 

 of proof for this possibility lies upon 

 those who would wish to verify the 

 efficacy of this kind of selection. 



Second we must consider cross- 

 breeding—to take part in which the 

 pure lines must forfeit their pure con- 

 dition! The whole hybrid question is 

 not, however, part of our discus- 

 sion. 



Third we come to mutations, the 

 possibility of irregular changes in gen- 

 otype. To define them would be pre- 

 mature in the greatest degree. Their 

 existence in a greater diversity of 

 organisms must first be substantiated. 

 That they do so occur cannot be 

 doubted, in my opinion; I hope to 

 present specific, positive proof in a 

 later publication. No more will be said 

 here than that a mutation in a given 

 direction cannot be specifically iden- 

 tified strictly on the basis of offspring 

 of individuals which deviate irregu- 

 larly in that direction. 



At this point I must refer to the 

 ticklish problem of what might explain 

 the statement of de Vries that one fre- 

 quently observes "minus" variations 

 are predominant in newly discovered 

 genotypes— an event that not without 

 reason has aroused the skepticism of 

 biometricians. It is to be hoped that 

 the studies presented here will shed 

 some light on this problem, which per- 

 haps only appears to detroy the 

 boundary between fluctuating varia- 

 tion and mutation. 



(Postscript: In the final part of his 

 Mutationstheorie — which appeared 

 during the editing of this work— de 

 Vries (I.e., pp. 503-504) has shown in 

 a most ingenious way how in most 

 cases mutations are first expressed. 

 Therein lies an outstandingly impor- 



JOHANNSEN 



tant instance for the explanation of the 

 relationship just discussed.) 



Hugo de Vries has included in his 

 Mutationstheorie (vol. 1, p. 368 ff.) a 

 separate chapter concerning "Nourish- 

 ment and Natural Selection" in which 

 the consequences of a rich or scanty 

 nourishment by the maternal plant is 

 discussed. I have no doubt that actual 

 or imaginary differences in nourish- 

 ment occurring simultaneously with 

 the presence or absence of selection 

 would account for de Vries' example. 

 Also, the phase of ontogeny which de 

 Vries called the "sensitive period" is of 

 particular interest. I could cite a similar 

 phenomenon in my research material 

 only with the greatest difficulty. It 

 should be understood that it is not my 

 intention to try to explain through the 

 "principle of pure lines" and nothing 

 else all the differences in characteristics 

 which selection in conjunction with 

 extreme or experimentally designed 

 habitats might produce. In this regard, 

 which is of strong interest to Neo- 

 Lamarckians, there is still very much 

 research to be done— and certainly 

 with the use of genuinely pure lines 

 as a research material. 



My primary purpose is to shed some 

 light on the Galtonian regression be- 

 tween ancestors and descendants, and 

 I believe that my material, which evi- 

 dently has its natural peculiarities 

 analogous to those of Galton, has its 

 value as a basis for analysis of the 

 Galtonian laws applying to popula- 

 tions. My statements conflict neither 

 wiih Galton's statements nor with 

 those of de Vries. 



If my investigations are sound, and 

 their significance is grasped beyond the 

 special case here discussed, the general 

 results of this work would form a not 

 unimportant support for the modern 

 concepts of Bateson and de Vries on 

 the great significance of "discontinu- 

 ous" variations, or "mutations," for 



