310 A. FRANKLIN SHULL 
Just such a progressive change occurs in daphnians (Wol- 
tereck, ’11), but here the number of sexual individuals increases 
with the age of the line, instead of decreasing as in Hydatina. 
That the change should be in the opposite direction in the roti- 
fers and daphnians need hardly surprise one, since other phenom- 
ena are reversed in the two groups. For example, late females in a 
family of daphnians produce more sexual daughters than do their 
older sisters, while late females in a family of Hydatina produce 
fewer male-producers (sexual females) than do the early females. 
The progressive decrease in the proportion of male-producers 
may also have a bearing on pure line* work in general. So far 
as we know there is no method by which parthenogenesis may 
change the genotypic constitution of a line, yet parthenogenetic 
lines of Hydatina do change. May not pure lines suffer progres- 
sive change, notwithstanding they are composed of homozygous 
individuals? If so, there can be differences between two pure 
lines having the same genotypic constitution, even when both are 
reared under the same external conditions. If this were found 
to be true, it would not invalidate the conclusion that pure line 
differences exist, but would modify our explanation of them and 
their apparent behavior in inheritance. 
How this progressive decrease in the proportion of male-pro- 
ducers is brought about is not known. At first, it seemed that 
the proportion of male-producers might be determined by the 
vigor of the parthenogenetic line. The view that long-continued 
reproduction, whether bi-sexual, parthenogenetic, or vegetative, 
without the introduction of new ‘blood’ in crosses, is detrimental 
to vigor, is often expressed, even if not always correct. Such 
a loss of vigor seems to occur in Hydatina, as evidenced by the 
decrease in the size of family in successive parthenogenetic gen- 
3 In an earlier paper I have spoken of a series of parthenogenetic generations in 
Hydatina senta as a pure line. While parthenogenetic species do not meet the 
requirement of Professor Johannsen’s definition of a pure line, there seems to be 
no abuse of the fundamental conception of pure lines in applying the term to par- 
thenogenetic species. The term klon, orclone, used by plant geneticists to denote 
vegetatively produced varieties, can hardly be used for Hydatina, since there is a 
wide step between parthenogenesis and vegetative reproduction. Under these 
circumstances I have preferred to use the term parthenogenetic line in the present 
paper. 
