No. 517] 



PARENTAL MODIFICATIONS 



7 



Assuming that the latter had been modified to the extent 

 x, the former, according to hypothesis, would be found to 

 be modified to the extent x -J- x/n ; i. e., the effect of the 

 conditions would have been cumulative. 



Xow. as a matter of fact, I have attempted both of 

 these tests upon rather a large scale ; but I have not yet 

 found the second one to be practicable, owing to the diffi- 

 culty, without special facilities, of repeating precisely the 

 same temperature conditions during the lives of two suc- 

 cessive generations. But the first test proved to be prac- 

 ticable, and has yielded the results which are summarized 

 in the ensuing paper. Since the full data are to be pub- 

 lished elsewhere in the course of the coming year, the 

 omission of certain important portions of the evidence 

 will, I trust, be condoned. 



The parents of the mice to be discussed had been 

 divided, at the commencement of life, into two lots, which 

 were reared in separate rooms, differing widely in tem- 

 perature. 4 Prior to the time of the pairing, they had 

 been exposed to these conditions for an average period of 

 about six months. For reasons which I shall not here 

 state, the two contrasted groups ("warm" and "cold" 

 lots) were not transferred to a common room before 

 pairing. 8 Indeed, the females were not removed to such 

 a room until the time that each was discovered to be 

 pregnant. The discovery was made, on the average, 

 about five days before the birth of the young, ?'. e.. about 

 two weeks after the actual commencement of pregnancy. 

 This circumstance is of possible importance in interpret- 

 ing the results obtained. On that subject, more anon. 



From the time that pregnancy became apparent, the 

 mothers of the two lots were kept in the same room and 



ularly guarded against. P 1 



