202 Experimental Zoology 



conditions, be awakened to activity, we seem to get further insight 

 into the conditions. That ahemate or contrasted characters 

 may exist in a single germ is well recognized. A further exam- 

 ple will show the probabihty of this explanation. If individ- 

 uals of two races, having different "modes," are crossed, the 

 offspring will often show a tendency to give a curve with two 

 summits — one for each parent mode. The characters of both 

 parents are present in the offspring and sometimes one, some- 

 times the other, dominates. These, however, are special cases 

 and are by no means to be taken to illustrate the usual effects 

 produced by crossing two races. 



Selection of Fluctuating Differences 



We come now to the question of inheritance in regard to fluc- 

 tuating variation. Ordinarily, succeeding generations show 

 the same variations. This apphes when the pairing of the indi- 

 vidual is left to chance alone, and when the external conditions 

 are similar. But the outcome is different if individuals show- 

 ing the same kind of fluctuation are picked out or selected and 

 inbred. The most striking results are produced, of course, when 

 individuals showing one extreme or the other are utihzed. Sup- 

 pose, for example, we select two individuals, standing near the 

 same outer hmit of the curve — two individuals, let us say, 

 showing the fluctuation in question developed almost to the 

 greatest degree ever found. If the offspring of this pair be 

 measured, it will be found, in most cases, that the average has 

 been raised in the direction selected. The curve has moved 

 toward the variation selected in the parents. In other words, 

 and this is the important point, the average of the individuals 

 is now higher than before, i.e. there are now more individuals 

 than in the preceding generation with the character in question 

 better developed. By selecting again individuals from the same 

 extreme, the average can again be transferred in the same direc- 

 tion. As the process is continued, however, the average gain 

 soon decreases, and finally stops without ever having transgressed 

 the outer limit of fluctuation found in the first instance, provided 



