modification of Germinal Constitution of Organisms 245 



selected parents there came a general population less and less variable, 

 and a greater or less number of highly divergent forms beyond the 

 normal range of variability of the species. These latter we shall 

 consider in another place. 



In this series of cultures a normal parent stock has been subjected 

 to artificial selection aided by powerful environmental stimuli, both 

 having the production of the same end in view. The results, however, 

 were a keen disappointment; the inability to produce by selection and 

 powerful environmental influences a race much beyond the normal 

 limits of variability of the species might easily be taken to indicate 

 the impotency of selection. The ease with which the beetles moved 

 back toward the mode when selection was no longer practiced and 

 the conditions of existence became modal, when joined to the data 

 of place and geographical variations, allows only of the conclusion 

 that while differently colored races and modifications of this organ- 

 ism occur in nature and are produced in experiment by artificial 

 selective processes and local environmental influences, such modi- 

 fications are limited by the natural limits of variation of the species 

 and persist only as long as the maintaming processes are present 

 and are utterly incapable of existence under adverse conditions, 

 reverting to the species type. That is, artificial selections or local 

 influences are able to modify, and to a certain extent create, races 

 founded upon those variations which are ordinarily killed off by 

 natural selection; but in the creation of such races we really have 

 two forces — a species tendency and a local (or artificial in experiment) 

 — acting against one another, with the result that selective divergence 

 to a certain limit is attained, but beyond that the racial divergence is 

 slow or entirely stopped. When the local or artificial selection is re- 

 moved the species selective tendency causes a regression to the type 

 of the species. It may be objected that my experiments do not cover 

 a sufficiently long series of generations to have accomplished the 

 result intended, and this may be true; but selection is a powerful 

 formative factor and works rapidly up to a certain limit, and this has 

 been abundantly proven by plant and animal breeding for fifty )'cars. 

 Why should it not also be able to establish a race on permanent 

 footing with the same rapidity ? It is known from the rearing of 

 domestic animals and plants that constant selection is necessary 



