116 Francis B. Sumner 
outset by a system of clipping the right ear and various fingers and 
toes of the right foot. Rather contrary to my expectations, the 
alien offspring were accepted by the mothers as readily as their 
own, so that little if any loss of life resulted from this procedure. 
A certain proportion of deaths occurred here as always during 
the rearing of these mice. The number of deaths in the 
warm room during the first six weeks was 6, giving a mor- 
tality of about 9 per cent. The number dying in the cold room 
was considerably greater, being 13, or about 20 per cent of the 
lot. During the next month of life no deaths occurred among 
the warm room lot, while 6 were recorded in the cold room; but 
scarcely any further deaths from natural causes occurred during 
the next four months, 1. e., until the end of the experiment. ‘The 
cold room individuals, throughout the earlier part of their life, 
at least, were much less active than those in the warm room. 
During the first few weeks they kept to their nests almost con- 
stantly. Nevertheless, when mature, they were of decidedly 
better appearance than the warm room lot, and, when paired, 
they reared a much higher percentage of offspring. It must be 
added, however, that the reproductive capacity of both lots was 
found to be distressingly slight—so slight, in fact, as to render 
futile any attempt to make a satisfactory test of the transmissi- 
bility of the modifications which had been produced. Among 
the 21 females in the cold room lot, 31 pregnancies are recorded for 
the 15 weeks during which they were kept with the males, while in 
the aggregate only 48 young were reared to the age of six weeks. 
Indeed, the majority of the litters either consisted entirely of still- 
born young, or of ones which died during the first few days after 
birth. In other cases, the young were apparently born healthy, 
but the mothers seemed unable to suckle them or perhaps lacked 
the instinct to do so. With the warm room lot the case was even 
worse. Of the young resulting from 50 pregnancies (doubtless 
over 200) only 35 individuals, or about 15 per cent, survived to the 
age of six weeks, while in the great majority of litters all the indi- 
viduals died either before or shortly after birth. I am still almost 
wholly at a loss to account for this failure of the powers of repro- 
duction. The mice were paired rather too young, it is true, being 
