An Experimental Study of Somatic Modifications etc. 319 
and thus the experiment was brought to a close. It is not intended, 
however that the matter shall rest here. 
In order to fairly test the heritability of somatic modifications, 
individually acquired, the following conditions should, I believe, be 
realized: 1) We must select for experiment such an organism and 
such a physical agency that the latter may modify the former without 
directly influencing the germ-cells. 2) We must discover readily 
measurable, quantitative changes in the parent generation before we 
can hope to test the reappearance of such changes in the offspring. 
Most of the past experiments in this field have been rendered 
inconclusive by a failure to conform to the first of these conditions. 
Thus the congenital effects of temperature and humidity upon insects, 
which have been described by Standfuss^), Fischer 2), Tower 3) and 
others, are equally well explained as the result of an immediate mo- 
dification of the » germ-plasm « by the external stimulus itself. The 
same is true of the enduring effects of special feeding, cumulative 
from generation to generation, such as have been described by Pictet*) 
for Lepidoptem and by Houssay^) for fowls. Indeed this postulate 
of a » direct effect upon the germ-plasm « has been freely used by 
Weismann and his followers as a cheap and easy way of disposing 
of most of the experimental evidence which has been brought forward 
in favor of the inheritance of acquired characters. But it scarcely 
seems applicable to any effects which may be found to result from 
the action of temperature upon a mammal. For differences of ex- 
ternal temperature, as such, manifestly cannot reach the germ-cells of 
a warm blooded animal unless during the first few days after birth 6). 
1) Zur Frage der GestaltUDg und Vererbung auf Grund achtundzwanzig- 
jilhriger Experimente. Leipzig 1902. 
2) AUgemeine Zeitsclirift fiir Entomologie. 6. 1901. 7. 1902. 
3) An Investigation of Evolution in Chrysomelid Beetles of the Genus 
Leptinotarsa. Carnegie Institution. 1906. p. 320. pi. 30. 
4) Memoires de la Societe de Physique et d'Histoire naturelle de Geneve. 
35. 1905. 
5) Archives de Zoologie Experimentale et Generale. 4^^ serie. VI. 1907. 
6) Pembrey (Journal of Physiology. Vol. XVIII. 1905) found the body 
temperatures of adult mice to remain constant at widely different external tem- 
peratures. In the young, however (under ten days old), the internal tempera- 
ture was found to vary with that of the atmosphere. I'hese experiments should 
be repeated with the aid of more delicate instruments than ordinary mercurial 
thermometers. At present it is, of course, open to the Weismannian to contend 
1) that very slight differences of internal temperature may actually be produced 
in the parents and may be responsible for any effect upon the succeeding 
