An Experimental Study of Somatic Modifications etc. 323 
proper, may therefore have played a part in the results produced. 
For present purposes, however, it matters little whether one or both 
of these factors was concerned. 
Each entire brood of mice with its mother ^j, was transferred as 
soon as it was received from the dealer to one or the other of these 
experimental rooms. No selection was exercised in thus disposing of 
the mice, the broods being chosen at random 2). 
In all 98 mice, representing 22 broods, survived to the age of 
42 days in the warm room; 88 mice, representing 23 broods sur- 
viving to this age in the cold room. During the ensuing months there 
were, as commonly happens, a considerable number of deaths. At the 
time of the final pairing (May 2) 39 warm-room females and 34 cold- 
room females were still living. The males, with the exception of 
those which were selected for breeding purposes ^j, had previously 
been killed. 
The following table from my early work (1909, p. 138) summarizes 
the results of the measurements made upon these mice at the age of 
six weeks. The diagram (Fig. 2) shows the differences in tail length 
between the warm-room and cold-room individuals (both sexes), these 
being divided into groups according to weight. 
Series of 1908 — 1909 (parents): Weight and tail length 
at six weeks. 
Weight 
Tail length 
Males 
Females 
Males 
Females 
Warm 
Cold 
Warm 
Cold 
Warm 
Cold 
Warm 
Cold 
(55) 
(50) 
(43) 
(38) 
(53) : 
(47) 
(42) 
(35) 
Mean ..... | 
12.604 
13.180 
12.663 
11.889 
67.19 
60.11 
68.95 
59.49 
± 0.238 
±0.241 
± 0.217 
± 0.195 
±0.55 
±0.51 
±0.48 
±0.53 
Standard de- 1 
3.119 
2.522 
2.107 
1.783 
5.98 
5.20 
4.61 
4.63 
viation | 
± 0.201 
±0.170 
±0.153 
± 0.138 
±0.39 
±0.36 
±0.34 
± 0.37 
1) In a large proportion of cases before birth (see above). 
2) It would doubtless have been a somewhat more scientific method of 
procedure to divide each litter into two portions, for the warm and cold rooms 
respectively, thus ensuring the presence, in these two rooms, of mice having 
the same parentage. This indeed had been done in the series of the preceding 
year, but such a procedure naturally involved considerable difficulties. I have 
therefore acted upon the supposition that the use of a sufficiently large number 
of broods would minimize the possibility that the two contrasted lots dilfered 
appreciably in their mean hereditary constitution. 
3) The principle of selection was as follows: that male was chosen from 
21* 
