202 



THE AMERICAN NATURALIST 



[Vol. XL VI 



genotype, but the genotype is not a function of ontogenesis. The idea 

 of evolution by continuous transitions from one type to another has 

 imposed itself upon zoologists and botanists, who are examining chiefly 

 shifting phenotypes in very fine gradations. There is such a continu- 

 ity in phenotypes but not in the genotypes from which they spring. 

 All degrees of continuity between phenotypes may be found, but real 

 genetic transitions must be distinguished from the transitions which we 

 find in museums. 



Genotypes, it is true, can only be examined by the qualities and re- 

 actions of the phenotypes. 



Such examination shows that within pure lines — if no new muta- 

 tions or other disturbances have been at work — there are no geno- 

 typical differences in the characters under examination. The only real 

 discontinuity is that between different genotypes. The mutations ob- 

 served in nature have shown themselves as considerable discontinuous 

 saltations. There is no evidence for the view that mutations are prac- 

 tically identical with continuous evolution. In pure lines no influence 

 of special ancestry can be traced; all series of progeny keep the geno- 

 type unchanged through long generations. Discontinuity between 

 genotypes and constant differences between the genes show a beautiful 

 harmony between Mendelism and pure line work. 



Selection will have no hereditary influence in changing genotypes. 

 Even the selection of fluctuations in pure lines is ineffective to produce 

 a new genotype. 



Heredity may thus be defined as the presence of identical genes in 

 ancestors and descendants, or heredity stands for those properties of 

 the germ cells that find expression in the developing and developed 

 phenotype. 



Similarly Jennings observes: 20 What distinguishes the different 

 genotypes, then, is a different method of responding to the environ- 

 turn!. And this is a type of what heredity is; an organism's heredity 

 is its method of responding to the environmental conditions [p. 84]. 

 ... It appeared clear, and still appears clear, that a very large share 

 of the apparent progressive action of Selection has really consisted 

 in the sorting over of preexisting types, so that it has by no means the 

 theoretical significance that had been given to it [p. 88]. ... I had 

 hoped to accomplish this myself, but after strenuous, long-continued and 

 hopeful efforts, I have not yet succeeded in seeing Selection effective 

 in producing a new genotype. This failure to discover Selection re- 

 sulting in progress came to me as a painful surprise, for like Pearson 

 I find it impossible to construct for myself a " philosophical scheme of 

 evolution," without the results of Selection and I would like to see 

 what I believe must occur [pp. 88-89]. ... It would seem that the 



we find out how this happens, then such Selection between genotypes 



