Xo. 517] 



1\ L REX I A L M ODIFICATIONS 



13 



the same temperature conditions as were these parents. 

 To determine the extent of the differences, we shall 

 consider, not the differences between the gross aver- 

 ages, for reasons already stated, but the average differ- 

 ence, within each size group, between the "warm" and 

 the "cold" figure. These mean differences, representing 

 the extent of the modification shown at the age of six 

 weeks by mice directly influenced by temperature are : 



Corresponding figures for the warm-room and cold- 

 room descendants with which we have been dealing are: 



Comparing these two sets of figures, we find that the 

 difference in tail length is 13 per cent, as great in the 

 second case as in the first; the difference in foot length 

 is 26 per cent, as great, while the difference in ear length 

 is 63 per cent, as great! These figures are not offered 

 as expressing, with even a rough degree of approxima- 

 tion, the proportional part of these parental modifications 

 which is handed on to the offspring- even granting that 

 such a transmission occurs. The relative magnitude of 

 these three percentages is particularly surprising, in view 

 of the fact that the tail is the organ which responds most 

 decidedly to the temperature differences, while the ear 

 has been shown to be least affected. 1 1 It might be argued 

 that the very plasticity of a part, which makes it so re- 

 sponsive to outside influences, might render it correspond- 

 ingly ill adapted to retaining such impressions perma- 

 nently. 12 Such speculations are decidedly premature, 



Tail 

 Fool 

 Ear . 



.9.710 



.0.2021 



