21G ERNEST WAimKN. 



in which the animals were living. The slope of the line 

 a a' a" (Fig. 4) is not very different from that of a a'; and 

 since the series ''a" and "a" were living under fairly 

 similar conditions^ this fact would seem to point to the cha- 

 racter being inherited. The line h h' h" h'" shows a slight 

 downward tendency in the first generation, but in the second 

 and third generations there is a small rise above the parental 

 mean, which happened to be somewhat low. The mean spinal 

 length in the three generations was 196 thousandths, and 

 the parental mean was 194 thousandths; thus in the water 

 that was continually being changed the spine showed no 

 tendency to diminish. 



The relative length of the spine sinks as the animal grows ; 

 and since under favourable conditions Daphnia continue to 

 grow as long as they live, the mean never becomes quite 

 steady. Consequently the means given in Table V w^ould 

 only be absolutely comparable if all the animals had been 

 measured at one particular size. However, between the 

 limits of size employed the change in the mean would be 

 small, and if a connection could have been applied, it would 

 only have made the effects of the condition of the water still 

 more apparent. 



In the "a" series, where no fresh water was added, 

 twenty individuals grew to an average size of 2*621 mm. in 

 800 c.c. of water; this gave an average bulk of 40 c.c. of 

 water for each animal. The average diminution of spine was 



— — = 70 thousandths of body-length. 



Under the somewhat different conditions in the " a" series, 



where rain water was added, twenty- seven individuals grew 



up to a size of 2' 704 mm. in an average bulk of 30 c.c. of 



• ^ , . 276-218 



water, and the average lessening of the spine was — fAnn — 



= 58 thousandths. 



Thus quite roughly it may be said that if a newly hatched 

 Daphnia be allowed to grow up to "adult" life in 40 c.c. of 

 freshly drawn water, its spine would on the average be 



