Effect of Selection 68 



for evolution, for if no such changes occur, there would 

 appear to be no such thing as evolution. 



I made long continued attempts to change the size in 

 such a race of Paramecium, by selecting on the one hand 

 large individuals; on the other hand small ones (Jennings, 

 1908). No effect was produced. Large parents and small 

 parents, if they belonged to the same race, produced progeny 

 of the same size. Ackert (1916) has recently repeated these 

 experiments, with the same results. 



I tried also to produce from a single race stocks differing 

 in their rate of reproduction, but these attempts, like those 

 to change the size, met with no success. Each race seemed 

 permanent in its size and rate of reproduction. 



Jollos (1913) attempted by selection in Paramecium to 

 obtain stocks that differed in their resistance to heat and 

 to chemicals. He found this very easy when he began with 

 a wild population containing many diverse stocks; all he 

 had to do was to isolate the differing stocks. But when he 

 began with a single stock, he found that he could not by 

 selection get stocks diverse in their resistance. Throughout 

 the ordinary multiplication, all the individuals, like identical 

 twins, remained just alike. 



Similar results have followed many other attempts to 

 change by selection the inherited characteristics of a single 

 stock. Barber (1908) made long continued attempts to 

 change in this way the characteristics of pure stocks of 

 bacteria and of yeasts. He found a very few cases of sud- 

 den mutation, such as occur with similar rarity in higher 

 organisms ; of these we shall speak later. But, as a rule, any 

 differences found within a single race were not inherited; 

 the races were permanent. Wolf (1909) made repeated and 

 long continued attempts to change through selection the 

 colors in pure races of the red bacterium, Bacillus pro- 

 digiosus ; but he could produce no change in this way. 



