318 
Francis B. Sumner 
I have already dwelt upon the significant fact that these artifi- 
cially produced differences are just such as have long been known 
to distinguish northern from southern races of mammals. This fact 
»will be taken by some as evidence that these differences in nature 
are likewise , ontogenetic' or acquired independently by each indi- 
vidual. Conversely, 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 demon- 
strated by the present experiments* i). Neither conclusion is of course 
warranted until we have convincing evidence for or against the trans- 
missibility of this class of modifications. It has accordingly been my 
chief motive throughout the course of these experiments to obtain 
evidence of this sort. And it is the object of the present paper to 
report such data as I have thus far obtained 2). 
I trust that no apology is necessary for the presentation of re- 
sults which have been derived from a single generation of offspring, 
comprizing only a few hundred individuals. I need only point out 
to the reader that this meagre showing is the fruit of monotonous 
drudgery 3) occupying most of my spare time for over three years, 
during which period my best energies were unavoidably devoted to 
a quite different task. Without special facilities (and none were at 
my disposal) it was obviously impossible to obtain the necessary differ- 
ences of temperature except during the winter months. If one ex- 
periment failed, it was necessary to wait until the following winter 
before I could begin anew. Indeed, it was not until the close of the 
third winter that I obtained a generation of offspring which fulfilled 
the requirements of the test^). And unfortunately it was not possible, 
even in this case, for me to keep these animals long enough for fur- 
ther breeding. They were all killed at the age of about 3 1/2 months, 
1) Op. cit. p. 152. 
2] A preliminary account of these data was published in the American 
Naturalist. January 1910. 
3) Any one who has had experience with animal breeding knows that the 
mere daily care of several hundred mice requires not a little time, especially 
when one is unassisted. Even this labor has been trifling, however, in com- 
parison with that required for the measurements and the subsequent com- 
putations. 
■*] I. e., which contained a large enough number of healthy animals born 
nearly or quite simultaneously. 
