Karl Pearson 
373 
— yet he has given his approval and sanction on two separate occasions* to a 
memoir by Elise Hanelf on asexual propagation in Hydra grisea. We have 
thus a very fair test of what his judgment on a statistical proof of "pure line" 
heredity is worth. 
I propose to discuss at some length Hanel's paper, because it seems to me 
that no definite conclusions as to " pure lines " can be obtained by investigations 
of this type, until the fundamental features of the research are modified and 
the observations treated with an adequate theoretical knowledge of statistics. 
Hanel begins with a very careful investigation of the growth and environmental 
changes in the char acter selected — the number of tentacles in Hydra grisea. There 
is general agreement with Parke's results that the number of tentacles changes 
with age, size, food and place of culture. Differences in these factors can produce 
very considerable differences in individuals, and differences in the averages of diffe- 
rentially treated groups, which can amount to as much as 0"5 to 0"8 of a tentacle. 
These are precisely of the order of the average hereditary differences. Thus : 
Parent 
Offspring 
Number of 
Number of 
Number of 
Average of 
Individuals 
Tentacles 
Individuals 
Tentacles 
9 
6 
364 
6-943 
9 
310 
7-296 
.4 
8 
166 
7-344 
4 
9 
12.5 
7-383 
* "Heredity, Variation and Evolution in Protozoa" ii. , Proc. Amer. Phil. Soc. Vol. xlvii. 1908, 
p. 521, Jennings definitely asserts that Johannsen in 1903 showed that in beans and barley selection 
within the pure line had no effect. He then proceeds to state that Hanel has found the same state of 
affairs— no selection within the pure line will produce an effect — in Hydra. In a second very dogmatic 
paper "Heredity and Variation in the Simplest Organisms," The American Naturalist, Vol. xliii. 1909, 
p. 332, Jennings writes: "Work with 'pure lines' — where no intercrossing of races or individuals 
occurs — is possible with few organisms, and little of it has been done. In the few investigations 
carried on in this way, the same conditions have been found that we have set forth above for Para- 
mecium. They were first shown by Johannsen to hold for beans and barley, and later by Elise Hanel 
for Hydra. The fact that there exist diverse races, tending to breed true, has of course been shown for 
many species, but in most cases it is diflficult to maintain pure lines, and thus to absolutely demonstrate 
the relations above set forth, as has been done for beans, barley, Hydra and Paramecium." The italics 
are mine. He also cites Barber " On Heredity in certain Mioro-Organisms," The Kansas University 
Science Bulletin, iv. 1907, pp. 1 — 48. Now several comments are needed here. Jennings directly 
overlooks the researches of the Biometric school, which was first to work in the field of pure lines, and 
actually found (see Warren's papers, R. S. Proc. Vol. 65, 1899; Biometrika, Vol. i. p. 128, 1901) 
that variations within the line were inherited. The same result appears to flow from Johannsen's and 
Hand's own experiments, but the characters they have chosen are so influenced by environment 
that they have not observed the fact. It does not flow from Jennings' or Barber's experiments because 
they have not demonstrated that heredity exists at all for the characters selected. In fact Jennings 
repeatedly (and Hanel also) illustrates how markedly environment dominates the characters taken as a 
measure of heredity. Jennmgs then assumes that by selecting a character, the heredity of which he has 
never demonstrated, he can reach a general and " absolute demonstration " of the truth of the theory of 
pure lines ! 
"Vererbung bei ungeschlechtlioher Fortpflanzung von Hydra grisea," Jenaische Zeitschrift, 
Bd. XLIII. 1908, pp. 322—372. 
Biometrika vii 48 
