No. 530] TEMPERATURE ON GROWING MICE 



95 



present case would be this. Individuals belonging to a 

 single pure line would probably respond with much 

 greater uniformity to the effects of an environmental 

 change than would those belonging to a composite stock, 

 consisting of a number of lines. It is quite conceivable 

 that among these last some would respond in a much 

 greater measure than others. Or indeed some might not 

 be affected at all. But here the much-scorned "mass 

 statistics" would reveal the mean tendencies of the two 

 lots, and the resulting data, though confessedly capable 

 of further analysis, would be none the less valuable. If 

 it be objected that the differences between the two aver- 

 ages may be due to the presence in one or both of the con- 

 trasted lots of a few "mutants," while the remaining 

 individuals may not have been affected at all, I will only 

 point out, as above, that the frequency distributions are 

 directly opposed to such an assumption. 



Having produced modifications of the sort mentioned, 

 it remained to be seen whether these effects persisted 

 beyond the generation immediately influenced. . . . (This 

 part of the discussion, including an account of the method 

 employed, the results, and certain of the possibilities of 

 interpretation, I have thought it best to omit here, in view 

 of the fact that I have covered practically the same 

 ground in statements already published. I will merely 

 note that the offspring of warm-room and cold-room mice, 

 although themselves reared under identical temperature 

 conditions, presented differences of the same sort as had 

 been brought about in their parents through the direct 

 effect of temperature, viz., differences in the mean length 

 of tail, foot and ear.) 



There remain two principal alternative explanations, 

 which are not wholly distinguishable from one another, 

 and neither of which admits of being stated ew<»pt in 

 rather vague terms. 



One of these is the assumption that the changes under- 

 gone by the parent body are in some way registered in the 

 germ cells, so as to be repeated, in a certain measure, in 



