218 A Biometrical Stvdy of Conjugation in Paramecium 
were, throughout the epidemic, present in relatively small numbers. It was 
feared that if large enough samples were taken to ensure statistically adequate 
numbers of conjugating pairs the culture would either be exhausted, or its balance 
so disturbed that the Paramecia would rapidly disappear from it. Accordingly 
the plan adopted was to take a single drop of culture fluid on the slide at a time, 
search carefully through it, and if a pair of conjugants were found, kill and measure 
them in the manner presently to be described. By working in this way as 
rapidly as was consistent with accuracy, it was possible to measure a reasonable 
sample in two or three working days. 
Between each of the different samplings made in this way roughly a week 
elapsed. It would, of course, have been possible to have measured, even on this 
plan, somewhat larger samples than those actually taken, but it was hardly 
practical. After searching and measuring under high pressure for three days 
of about eight working hours each, one's eyes were so fatigued by the strain 
that further work without an intervening period of rest was almost an impossi- 
bility. The samples taken are, moreover, quite adequate, I think, to show the 
prevailing condition in the culture at the time. 
The measurements taken, then, fall into time groups, as shown in the following 
table. 
Series 
Dates of Measurement 
Number of 
Conjugants 
measured 
Number of 
Non-Conjugants 
measured 
A 
C 
D 
E 
August 15th P.M. — August 18th noon, inclusive 
„ 24th A.M.— „ 26th P.M. „ 
„ 30th 
September 6th 
105 pairs 
101 „ 
16 „ 
210 
202 
32 
132 
A word of explanation is needed regarding Series I) and E. These series 
would have been as large as the others but for the fact that there were but few 
individuals in the culture to be measured. By August 30th the epidemic of 
conjugation was practically over. The 16 pairs of Series I) were all that could 
be found after a painstaking search lasting all day, and with two persons working 
separately. I was compelled then to do with this very short series. No reason 
for the ending of the epidemic was evident, so far as the observable cultural 
conditions were concerned. 
By September 6th, however, the case was somewhat different. During the 
week from August 30th to September 6th blue-green algae had begun to grow 
very rapidly in the culture, until at the latter date they had formed a more or 
less dense mat over nearly the whole surface of the culture. Concurrently with 
this increase in the algae the numbers of Paramecia had steadily and rapidly 
diminished, so that there were comparatively few survivors, and it was only with 
some difficulty that I got the 132 which were measured. These survivors were 
