220 A Biometrical Studf/ of Conjugation in Paramecium 
Worcester's formol-subliraate mixture*, consisting of a saturated solution of cor- 
rosive sublimate in 10 per cent, formalin. This fluid, when properly used, kills 
Paramecium without visible distortion or shrinkage f. It is the fluid which has 
been used in collecting the extensive series of Paramecium material mentioned 
above (p. 216). Some time has been spent in an attempt to measure the same 
individuals before and after killing, but it has not been possible to find a means 
which would keep them quiet enough to measure during life with anything 
approaching accuracy, which did not obviously distort them at the same time. 
This statement is made after considerable experience with the various substances 
which have been suggested for this purpose, such as, for example, thin gelatine 
solutions, etc. That whatever of shrinkage and distortion is produced by the 
use of the killing fluid is less than the errors of measuring I am certain, though 
at present I cannot give figures to prove this. Of course it is not to be under- 
stood that absolutely all the individuals so killed will not be distorted. Naturally, 
there will always be specimens which the killing fluid reaches only in a diluted 
state, but such specimens will be obviously recognizable as distorted, and of 
course will not be included in the records. 
The individuals were at once measured while still in the killing fluid. By 
this procedure any distortion which might arise from the action of diffusion 
currents was avoided. The search for conjugant pairs was greatly facilitated by 
the use of a mechanical stage. The microscope used was by Bausch and Lomb, 
and the actual magnification was that given by a | inch objective and a 1 inch 
ocular, with a tube length of 160 millimetres. The method of measuring followed 
was the same as had been previously used in a study of variation in ArceUa\. 
By means of a camera lucida the points to be measured were projected on cards, 
marked with a fine-pointed pencil, and subsequently measured with a sliding-arm 
caliper with a vernier reading to tenths of a millimetre. By multiplying the 
* Cf. Pearl, E. , Journal of Applied Microscopy, Vol. vi. p. 2451. 
t Note added Nov. 10. In bis criticism of this paper, Mr Lister {loc. cit.) says : "In the first place, 
the specimens have been preserved and fixed, a process which every practical biologist knows to be 
attended with distortion." By this statement Mr Lister can only intend to spread abroad as widely as 
possible an ex cathedra pronouncement that I am not a "practical biologist." During the last six 
years a large part of my time has been spent in the study of living and fixed Paramecia. From 
statements made in Mr Lister's note I am convinced that he has never made any careful or thorough 
study of Paramecium (cf. infra, p. 221). Yet, so far as I can see, for no other reason than that I happen 
to be in a very modest way a worker in biometry, Mr Lister says that I am not a "practical biologist," 
and with all the weight of his authority implies that my statement that my material was undistorted is 
not true. 
I know of no way to answer a criticism which simply denies the existence of a scientific con- 
science. Fortunately the innate courtesy of men of science generally makes such criticisms rare. I can 
only repeat what is stated in the paper above that the distortion produced by the method of killing 
I have used on Paramecium is well within the limits of instrumental error in the measuring, or in other 
words that the specimens were not visibly distorted. The truth of this statement I stand ready to 
demonstrate directly to anyone interested, by exhibition of living and killed specimens side by side. 
In the present state of development of micro-photographic technique it is unfortunately not a thing 
which can be demonstrated on paper. E. P. 
X Pearl, E., and Dunbar, F. J. : Bioiuctriku, Vol. ii. pp. 321 — 357. 
