78 SEVENTH REPORT. 



not furtlier manipulated. To one of the others (B) was added 2 parts 

 of an w/1 NaCl solution to each 98 parts of culture solution. To the 

 third (C) was added 1 part of m/1 cane sugar solution to each 99 parts 

 of culture fluid. The three vials (corked) were then put in the same 

 conditions with reference to light, temperature and other varying fac- 

 tors of the laboratory environment. So far then as it was possible to 

 control the matter experimentally the environmental conditions sur- 

 rounding the individuals in vials A, B. and C were identical except that 

 there was more NaCl in 7? than in A, and more .sugar in G than in A. 

 On account of the complex nature of the culture medium chemically 

 it is of course impossible to make any statement regarding the dhsolute 

 concentration of NaCI or sugar in the cultures. We merely know that 

 we are changing in a definite way a single factor in the chemical envir- 

 onment. 



At the expiration of a certain period of time all the Paramecia in 

 each of the three vials were killed with Worcester's formol-sublimate 

 fluid/ thoroughly washed and preserved in 4% formalin. This formol- 

 sublimate fluid we have demonstrated to be a practically perfect killing 

 fluid for infusoria, producing no measurable distortion, when used by 

 one who has had experience with it. With the specimens still remain- 

 ing in the formalin the length and breadth of each individual were 

 measured by means of a filar ocular micrometer. The measuring on 

 all the experiments is not yet completed. When finished we shall have 

 measurements of the length and breadth of between 7,000 and 8,000 

 individual Paramecia.^ 



We shall discuss here a part of the variation results for three of 

 the experiments which are designated in our notes as Experiments 1, 2, 

 and 7. In Experiment 1 the Parameci.a were in the experimental vials 

 100 hours; in Experiment 2, 200 hours, and in Experiment 7, 300 hours. 

 In Experiments 2 and 7 at the expiration of the first 100 hours there 

 was added to the B vial an amount of w/1 NaCl solution equal to what 

 was put in at the start. Or in other words, the concentration of the 

 solution in respect to NaCl was approximately doubled. In the same 

 way the concentration of the sugar in the G vial was doubled at the 

 end of the first 100 hours. In Experiment 7 a second addition of the 

 same amounts of NaCl and sugar to the B and G vials respectively 

 was made at the expiration of the second hundred hours. So then in 

 these two experiments, 2 and 7, we had a regular increase in the amount 

 of NaCl and sugar in the culture solution in vials B and G respectively, 

 occurring at 100 hour intervals. All of the 3.000 individuals here in- 

 cluded were the descendants of the same original ancestor. We shall 

 consider in this paper variation in length only. The point which we 

 wish especially to bring out here is the rather remarkable effect on 

 the variation constants which is produced by the addition of cane sugar 

 solution to the culture fluid. 



2. In order to bring together in convenient form the data on this 

 point we have prepared the following frequency distributions. The 

 first distribution (Table I) shows the variation in length of a random 

 sample of Paramecia taken from an ordinary laboratory culture set 



iPearl, R. Worcester's Formol-Sublimate Fixing Fluids. Jour. Appl. Micros. Vol. VI, No. 8, p. 

 2451. 



2The long and tedious task of making these measurements has been carried through with the utmost 

 care and patience by Miss Dunbar. R. P. 



