28 APPLICATION OF PKINCIPLES OF HEREDITY TO BEEEDING. 



collection at x — o:, rejecting all those to the right. By so doing it is evident that we 

 exclude all specimens of the two smallest races c and i, while preserving the majority 

 of the specimens of the larger races. Allowing these to propagate, we of course get 

 a mixture of the remaining larger races. Hence the mean size of the whole collec- 

 tion will be greater than at first. Selecting again those above the mean size of this 

 lot, we drop out another small race, and the mean of the collection as a whole again 

 rises a little. We are making good progress in the improvement of our species. By 

 taking successive steps of this character, dropping out the smaller races, first partly, 

 then completely, one after another, we can for a long time continue to improve by 



a 



X 



Fig. 1.— Graphic illustration of the range of fluctuations of each of the eight pure races of Paranieciuin 

 studied by Jennings. (Reproduced from the American Naturalist.) 



selection, but finally we reach a stage in which all but the largest race have been 

 excluded. Thereafter we can make no further progress. In vain we choose for 

 breeding the largest specimens of the lot; all belong to the same race, so that all pro- 

 duce the same progeny. Selection has come to the end of its action. * * * 



Selection here consists simply in isolating already existing races. It produces 

 nothing new. * * * 



Systematic and continued selection is without effect in a pure race, and in a mix- 

 ture of races its effect consists in isolating the existing races, not in producing any- 

 thing new. 



165 . 



