500 AUSTIN RALPH MIDDLETON 



individual variations is without effect in altering the hereditary- 

 characteristics. 



How are we to account for the discrepancy between the 

 present results, and those just mentioned? In Stylonychia we 

 are dealing with an organism which is large enough to be easily 

 handled and followed individually, so that no question can arise 

 as to the purity of the pedigrees (as sometimes occurs with 

 reference to Bacteria). In this organism the facts as to the 

 cumulative effect of selection are clear. 



We are of course dealing with a delicate physiological charac- 

 teristic, and this is perhaps more readily varied (even heredi- 

 tarily) than the characters examined by most- other investigators. 

 Further, it is perhaps true that hereditary changes are more 

 easily brought about in the Protozoa than in the more complex 

 organisms, for in Protozoa the 'apparatus of heredity' is in close 

 chemical contact with all the somatoplasm. 



But a certain feature of the experimental procedure in the 

 present case may have more importance than these conjectural 

 considerations. It has been possible in my work to make a 

 much greater number of actual selections (where plus and minus 

 cases were both present to choose from), than in most of the 

 work that has given negative results. And it has been found 

 that few selections give very slight results, and that a great 

 number are required to give any marked differences between 

 the sets. Thus, in my main experiment, on the average 39.86 

 plus selections were made in the fast-selected lines; 34.36 minus 

 selections in the slow-selected lines. The difference between the 

 two sets was thus the equivalent of some 74 selections extend- 

 ing through an average of 150 generations. This resulted in 

 the production of a constant average difference per line of 0.42 

 of one fission per day. 



Contrast with this great number of selections the six made 

 by Johannsen in obtaining his negative results with beans, the 

 three or four made by East with potatoes, the two made by 

 Winslow and Walker with bacteria, and similar small numbers 

 made by most other investigators along these lines; even indeed 

 the selection through fifteen generations made by Agar, in 



