Is there a Cumulative Effect of Selection ? 259 
During the last few years there has been a considerable amount 
of careful and critical experimental work done to test the method 
of action and limitations of continued selection in breeding. In 
particular reference should be made in this connection to the work 
of the Svalöf Experiment Station in practical plant breding;!) to the 
experimental work of Johannsen?2) with beans, and to the extensive and 
thorough investigations of Jennings®) on selection in Paramecium. 
The broad general result reached by these workers may be fairly 
stated as follows: From a mixed “general” population it is possible 
by a single selection to isolate pure strains (“pure lines,”’ “homozygote 
strains,” “pure races’) which will breed true and not revert to the 
mean of the general population from which they were isolated, 
regardless of whether further selection is practiced or not. It is 
impossible to demonstrate any cumulative effect of continued selection 
within the pure strain. Continued breeding from the extreme 
individuals of such a pure strain (‘‘fluctuating” variants) does not 
change the mean of that strain. From these considerations it follows 
that it will be difficult or impossible to make any definite and 
permanent change in the mean of a general population simply and 
solely by continued selection of extreme individuals, because in the 
vast majority of cases such individuals will be extreme fluctuating 
variants rather than mutants. Correns®) says (loc. cit. p. 51): ‘“Die 
Zuchtwahl, die kiinstliche sowohl wie die natiirliche, hat, auf die 
individuellen Varianten angewandt, jedenfalls keinen bleibenden Erfolg, 
wahrscheinlich gar keinen.” Jennings, in concluding his paper, makes 
a statement of similar import. He says (loc. cit. p. 522): ‘Certainly, 
therefore, until some one can show that selection is effective within 
1) For summaries of this work in languages more generally read than Swedish 
cf.: (a) Ulander, A., Die schwedische Pflanzenzüchtung zu Svalof. Eine kurze Dar- 
stellung. Jour. f. Landwirtsch. Bd. 54, 1906, pp. 105—124. Pl. II—-VII (b) 
Constantin, J. Le transformisme appliqué a l’agriculture. Paris (Alcan) 1906. 
Chap. IX. (Le Laboratoire de Svalöf et la Mutation), pp. 71—96. (c) De Vries, H. 
Plant Breeding. Chicago (Open Court) 1907, Chap. II (The Discovery of the Elementary 
Species of Agricultural Plants by Hjalmar Nilsson) pp. 29—106. 
?2) Johannsen, W. Über Erblichkeit in Populationen und in reinen Linien. 
Jena (Fischer) 1903. Pp. 68. 
%) Jennings, H.S. Heredity, Variation and Evolution in Protozoa. Il. Heredity 
and Variation of Size and Form in Paramecium with Studies of Growth, Enviromental 
Action and Selection. Proc. Amer. Phil. Soc. Vol. XVI, pp. 393—546. 1908. 
4) Correns, C. Experimentelle Untersuchungen über die Entstehung der Arten 
auf botanischem Gebiet. Arch. rass. u. gesellsch. Biol. Bd. I, pp. 27—52. 1904. 
17* 
