406 



THE AMEBIC AN NATURALIST [Vol. XL VI 



paper. The ups and downs of one series correspond 

 very closely with the nps and downs of the other series. 

 The mean killing of series 201 was 27.43 ± 1.75 per cent, 

 as against 77.51 ± 2.21 per cent, in nursery A, indicating 

 that the second winter was the milder one. 



The relation between the killing experienced by series 

 201 and series 202, during the 1910-11 winter, is ex- 

 pressed by the correlation table of Fig. 2. In this case 

 the 201 series is subject and the 202 series is relative. 

 The mean of the 202 series amounts to 6.43 ± 0.66. The 

 correlation in the table amounts to + 0.46 ± 0.07. As 



x 



Means of 202 Series 



X | 36 40 3 I 



§ 46 50 1 1 



| 51 55 11 



S 56 60 1 



indicated by the means of the two series, 27.43 and 6.43, 

 there was a remarkable apparent increase in hardiness. 

 This is partially expressed in the correlation table, but 

 in the table the most pronounced changes do not work 

 for a strong correlation. 



For instance, one Utah strain killed out 59.6 per cent, 

 in the 201 series, while its offspring killed but 6.2 per 

 cent, in the 202 series. This indicates a weak correlation 

 but a great increase in hardiness. There were but 3 

 instances of the 202 series killing more severely than the 



