620 
GENETICS: A. R. MIDDLETON 
followed individually, so that no question can arise as to the purity of 
the pedigrees (as sometimes occurs with reference to Bacteria). In 
this organism the facts as to the cumulative effects of selection are 
clear. 
We are of course dealing with a deHcate physiological characteristic, 
and this is perhaps more readily varied (even hereditarily) than the 
characters examined by most other investigators. Further, it is perhaps 
true that hereditary changes are more easily brought about in the 
Protozoa than in the more complex organisms, for in Protozoa the 
'apparatus of heredity' is in close chemical contact with all the 
somatoplasm. 
But a certain feature of the experimental procedure in the present 
case may have more importance than these conjectural considerations. 
It has been possible in my work to make a much greater number of 
actual selections (where plus and minus cases were both present to 
choose from), than in most of the work that has given negative results. 
And it has been found that a few selections give very slight results, 
and that a great number are required to give any marked differences 
between the sets. Thus, in my main experiment, on the average 
39.86 plus selections were made in the fast-selected lines; 34.36 minus 
selections in the slow-selected lines. The difference between the two 
sets was thus the equivalent of some 74 selections extending through an 
average of 150 generations. This resulted in the production of a con- 
stant average difference per line of 0.42 of one fission per day. 
Contrast with this great number of 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 investi- 
gators along these lines; even indeed the selection through fifteen gen- 
erations made by Agar, in Cladocera. It appears not at all inconceiv- 
able that in these organisms an equal number of selections, covering as 
great a number of generations, as were made in Stylonychia, would have 
given similar heritable effects. What all the work shows (and here my 
own is not in positive disagreement) is that heritable variations of con- 
siderable extent do not occur so frequently as was at one time supposed, 
so that a few selections are not sufficient for establishing a definite posi- 
tive effect. But negative results from a few selections are not sufficient 
for disproving the occurrence of heritable small variations which may 
be gradually accumulated. This indeed has been admitted by many 
of those who have obtained negative results. 
As a result of this work upon Stylonychia it is possible to give pre- 
