Effects of External Conditions 99 
myself with recounting some of the modifications produced during 
the lifetime of the individual. 
ENVIRONMENTAL CONDITIONS 
Early in life, in each experiment, the mice were divided into 
two lots, one of which was transferred to a room artificially heated,° 
the other to a room readily accessible to the winter atmosphere. 
The source of heat employed was a large steam coil which was 
ordinarily very effective. The room used throughout most of the 
work was, however, extremely pervious to draughts of air, and a 
strong wind from the proper quarter would sometimes bring down 
the temperature as much as 5 or 10° C. in the course of an hour 
or two. In the fall and early part of the winter, the steam was. 
available during the daytime only; commencing about January 
I, It was turned on night and day. ‘The cold room used in the 
two earlier series of experiments was situated in another part of 
the same building as the warm one. During much of the time 
a window was kept open in the former, though this was not neces- 
sary when the wind blew from the right direction, at which times 
the temperature would fall nearly to that of the atmosphere out- 
side. An unfavorable wind, on the contrary, frequently forced 
in warm air from other parts of the building. Thus in the case 
of neither of these rooms did conditions favor the maintenance 
of a very uniform temperature, and at times the fluctuations were 
rather violent.* Nevertheless the mean temperature throughout 
the experiments was very much higher in one room than in the 
other (Fig. 1), and the fluctuations resulted in little if any harm to 
the animals. Necessarily the humidity likewise differed to an 
enormous degree. In the cold room, the air was frequently near 
the saturation point. In the warm room, on the contrary, this 
5 Certain rooms in the U. S. Fisheries Laboratory at Woods Hole, Mass., not otherwise in demand 
during the winter months, were employed for this purpose. The material equipment has been pro- 
. vided almost wholly by the author himself. I must except a considerable number of cages kindly 
lent by Professor Morgan during the third winter of the work. 
® During the present winter my facilities have been much better. Through the kindness of the 
director, Dr. F. R. Lillie, I have had the privilege of locating my cold room in a small unheated build- 
ing belonging to the Marine Biological Laboratory. Here, therefore, the only fluctuations are those 
due to changes of weather. 
