GENETICS: A. R. MIDDLETON 
617 
parental reproduction inherited variations occur rarely or not at all, 
and that selection has no effect in altering racial characteristics. 
Most investigators, following Johannsen ('03, '09, '11), have found 
that 'pure lines' or 'clones' are hereditarily constant under selection. 
Johannsen's results were obtained with self-fertilized lines of beans. 
Similar ineffectiveness of selection has been found by Hanel ('08) and 
Lashley ('15) as to the number of tentacles in Hydra multiplying by 
budding; by Jennings ('08, '09, '10) for size in infusoria; by Barber ('07) 
(in the main), and by Winslow and Walker ('09), in bacteria; by East 
('10) in the vegetative reproduction of the potato; by Agar ('13 and 
'14) in Cladocera and aphids multiplying parthenogenetically; and by 
various other investigators on diverse organisms. Some discordant re- 
sults have been recorded, but most of these are ill-defined or uncer- 
tain; it is mainly in bacteria, with their immense difficulties for precise 
technique in pedigree work, that heritable variations or modifications 
have been described. The preponderance of evidence has been that in 
uniparental reproduction heritable variations do not occur (save as rare 
mutations of marked character), and that selection of sHght individual 
variations is without effect in altering the hereditary characteristics. 
These results have given origin to the concept of the Genotype 
(Johannsen), as a designation for the permanent heritable constitution 
of the race. The present paper deals with the inheritance of varia- 
tions and the effect of selection in the case of a most delicately poised 
and readily modifiable physiological character, the rate of fission of an 
Infusorian multiplying without conjugation. 
The questions here raised are: Can we, with respect to the character 
examined, get from a single genotype by selection two genotypes that 
differ characteristically from each other under identical conditions; and 
that retain these differences from generation to generation? Is selec- 
tion of small variations such as appear within the pure strain or clone 
an effective evolutionary procedure? 
For one hundred and thirty days two halves of a single clone (or set 
of individuals derived from the fission of a single parent) of Stylonychia 
pustulata were subjected to selection in opposite directions; this pro- 
duced a marked and steadily increasing difference in the average fission 
rate of the two halves. Expressing the excess of generations produced 
by the fast-selected lines as a percentage of the total number of gener- 
ations produced by both sets, the difference was 6.9% for the first thirty 
days; 12.8% for the next twenty days; 19.3% for the next thirty days, 
and 21.2% for the last fifty days. The records also show that for the 
fast lines the number of generations produced per line during the one 
