146 Francs B. Sumner 
from differences in temperature and to what extent from differences 
in humidity is not certain under the conditions of the experiments. 
10 Comparisons of earlier and later measurements upon the 
same animals show that there is a distinct tendency toward the 
reduction of these experimentally produced differences during sub- 
sequent growth, even when the conditions remain unchanged. 
11 A diminution in variability within each of the contrasted 
groups, during the course of growth, was shown to be probable 
for tail length, at least, and possible in the case of weight. 
In order to complete this summary, I will add by way of antici- 
pation: 
12 The modifications thus artificially produced are such as 
have long been known to distinguish northern from southern races 
of mammals. 
GENERAL DISCUSSION OF RESULTS 
In the following comment upon the results of these studies, 
I shall commence with the last statement in the summary. Itisa 
most significant fact that the experimentally produced differences 
which have been discussed in the present paper are found to be of 
just such a nature as are recognized by mammalologists and orni- 
thologists as distinguishing the northern from the southern rep- 
resentatives (individuals or geographical races) of some species. 
J. A. Allen has repeatedly called attention to the “marked ten- 
dency to enlargement of peripheral parts under high temperature 
or toward the Tropics.”’* Baird and other writers had previously 
made incidental mention “of the larger size of the bills of southern 
representatives of northward ranging species’? of birds, but 
Allen offers some detailed examples of this.* He concludes that 
“an increase in the length of the bill is most frequent in long-billed 
species, while in short-billed ones the increase is in general size, 
without material change in its proportions” (p. 231). A greater 
curvature sometimes accompanies the increase in length. Like- 
34 The influence of physical conditions in the genesis of species. Radical Review, I, 1877, pp. 108- 
140. (Reprinted with annotations in Annual Report of the Smithsonian Institution for 1905.) 
3 On the Mammals and Winter Birds of East Florida, etc. Bulletin of the Museum of Compara- 
tive Zodlogy, vol. 2, 1871, pp. 161-450, pl. iv—viil. 
