PAPERS ON ZOOLOGY AND PHYSIOLOGY 303 
SELECTION, REGRESSION, AND PARENT-PROGENY 
CORRELATION IN APHIS AVENAE FAB. 
H. E. Ewrne, Iowa STATE CoLLecE, AMES, [a, 
The pure line theory, at one time considered by many as be- 
ing established upon such a volume of evidence as to be al- 
most unassailable, is passing, like so many of our biological 
theories, through a period of adverse criticism. Especially 
have certain workers raised very serious objections to the 
earlier experiments in pure lines that gave negative results in 
regard to the effects of selection. These criticisms have re- 
cently been summarized by Jennings (1916), but the writer 
will here call attention to four of them which he intends to 
consider especially in this paper. These four criticisms are 
to the effect that: 
a. The characters used in the pure line work have been, 
for the most part, indefinite, and have not been structural 
characters that could easily be measured or counted. 
b. The characters used have been such as to be easily af- 
fected by environment. 
c. Not enough characters in any one line have been studied. 
d. Selections have not been carried on for a sufficient num- 
ber of generations in any one case to give a conclusive test of 
their effect upon somatic change. 
In regard to this last point Middleton (1915), who claims 
his results show positive effects of selection, asks us to com- 
pare with his 39 plus 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 Cla- 
docera.”’ 
Happily these criticisms so justly raised against the earlier 
experiments have been better met recently by the work of Mid- 
dleton, Ackert, Jennings and others, and the writer believes 
also by his own work with Aphis avenae Fab. But subjected 
to these more rigorous tests what results do we get? They 
