Effects of External Conditions 129 
at all) amounts to only about half a vertebra, i. e., it is not sufh- 
cient to make any appreciable difference in the tail length. In 
fact there is very little deviation from the mean throughout the 
entire series. It would be surprising indeed if the number of 
vertebra had been found to be altered by temperature conditions, 
first because this number does jnot in all probability vary very 
widely in most species of mammals, and, secondly, because the 
definitive division of the embryonic tissues into vertebrae is prob- 
ably complete long before birth. 
The present difference in ear length is not convincing, but 
foot length, has, with practical certainty, been affected by the 
tempeature conditions. 
It must be remembered that we are here dealing with mice which 
differ among themselves widely in size, and that neither the mean 
weight nor the mean body length is quite the same for the two 
groups. In Table 8 are presented the relative magnitudes of cer- 
tain characters. Here we have the mean ratios between the length 
of tail,ear and foot in each individual and the length of the body; 
likewise the ratios between the weight of the hair and the square 
of the body length. Inthe case of the ear and the foot, the vari- 
ability of the ratios is found to be much greater than that of the 
absolute measurements. ‘This is due to the fact that these parts 
vary but slightly as compared with the size of the animals. Indeed 
their length is remarkably constant throughout each series, irre- 
spective of the body length of the individual. 
The case of the Aair deserves a rather full discussion, since this 
is the character, in particular, whose modification may be supposed 
to be of an adaptive nature. It is seen from Table 6 that the mean 
weight of the pelage for the cold room individuals is 11.4 percent 
greater than that of the warm room individuals. The variability 
is very high, to be sure, partly because the animals vary much in 
size, partly because they actually vary in the density of their hair 
ton was not thoroughly denuded of muscles, etc., (alcoholic specimens were used) it was not always 
easy to distinguish the termination of the sacrum and the commencement of the caudal series, and 
an error of one or two vertebre perhaps resulted occasionally from this cause. In a few instances, 
some of the minute terminal vertebre were lacking, owing to obvious injury to the tail. In such 
cases, the figures have been enclosed in brackets and have not been included in making up the averages. 
