304 ILLINOIS ACADEMY OF SCIENCE 
are just as contradictory as before. Middleton (1915), who 
made 39.86 (average) plus selection with Stylonychia, comes 
to this conclusion: ‘Thus in Stylonychia, from a single clone 
of given genotype it is possible to obtain through long con- 
tinued selection during reproduction by fission, two sets 
(clones?) of diverse genotype, differing characteristically from 
each other in rate of fission under identical conditions; and 
retaining these differences from generation to generation. 
The selection of small variations, such as appear within the 
‘pure strain’ or clone, is then an effective evolutionary pro- 
cedure.” Jennings (1916), whose work with Difflugia corona 
was carried on in a very thorough manner, comes to similar 
conclusions in regard to the effectiveness of long-continued 
selection. He concludes his work with the following state- 
ment: ““Thus in general the investigation shows that in Diff- 
lugia corona a population consists of many hereditarily diverse 
stocks; and that a single stock, derived by fission from a 
single progenitor, gradually differentiates into such heredi- 
ta1ily diverse stocks; so that by selection marked results are 
produced.” Ackert (1916), however, working with Parame- 
cium obtains opposite results, and substantiates Jennings’ 
earlier findings with Protozoa, and thus gives further support 
for the pure line theory. In his summary he states: “In these 
experiments, in which nearly six thousand animals were meas- 
ured, the results indicate that selection within the progeny of 
a single Paramecium is without effect. Even when one of 
the animals selected was twice as long as the other, diverse 
groups failed to develop, each of the groups under comparison 
either having the same mean lengths, or the progeny of the 
longest Paramecium having the smaller mean length.”’ The 
writer’s results with Aphis avenae (Ewing, 1916) are in ac- 
cord with those of Ackert and the prevailing ideas of the 
pure line advocates. In my summary they are indicated by 
the following: “The results of this work with Aphis avenae 
Fab. are, we believe, sufficient to warrant the following gen- 
eralization: Fluctuating variations in a parthenogenetic pure 
line of Aphis avenae Fab., and presumably in all partheno- 
genetic pure lines, are in general not dependent upon germinal 
variations, and for this reason are not capable of increase or 
summation through the action of continued selection. Or to 
put it in another way: Fluctuating variability in a partheno- 
