152 Francis B. Sumner 
therefore, it is possible to separate these two factors 1n our experi- 
ments, we cannot state with any certainty to what degree each has 
been operative. A priori, it would seem, perhaps, that the changes 
in the mice have been such as could more reasonably be attributed 
to temperature. 
The fact that the same sort of differences as those which some- 
times obtain in nature between northern and southern species or 
varieties of animals have been produced by artificial conditions 
acting within the individual lifetime will be taken by some as evi- 
dence that these differences in nature are likewise entirely “onto- 
genetic’? or acquired independently by each individual. Con- 
versely, the neo-Lamarckian will perhaps argue—and with equal 
right—that here we have evidence that natural varieties and species 
have resulted from the accumulated effects of external conditions 
since the reality of such effects has been palpably demonstrated 
by the present experiments. Neither conclusion is justified by 
the facts before us. It remains to be settled experimentally (and 
thus only!) whether or not such modifications are transmissible. 
It has already been stated that no constant difference in size 
between the warm room and cold room individuals has been found 
to obtain throughout my series. Here, then, the reputed effects 
of natural climatic conditions have not been paralleled. It is 
quite possible that the cold was so severe during the early growth 
stages that some individuals were stunted. Indeed it has been 
pointed out for the 1907-1908 series that there was considerable 
mortality amongt the cold room lot in early life. Reference to 
the frequency polygons in Fig. 2 shows that there are two distinct 
modes among the cold room individuals; and I have determined 
that this is equally true of each of the sexes taken separately. 
The impression conveyed is that there are two pretty distinct 
groups, one of which was stunted by exposure to the cold, the other 
being favorably affected, so as even to surpass the warm room 
lot in size. It must be added, however, that no such effect is 
manifest in the 1908-1909 series. 
One of the most important general conclusions which seem 
warranted from an analysis of the foregoing results is the prin- 
ciple of the levelling down of experimentally produced inequali- 
