May, 1908.] Annual Wood-Increment of Acer rubrum L. 
347 
extracted. The result as corrected by a number representing 
the probable error is the standard deviation. We thus arrive 
at a value 1.87 mm. for the bog habitat, and 0.56 mm. for the 
woodlot habitat, which stands as a definite measure of varia¬ 
bility. It enables comparisons from year to year and between 
different localities, advantages which are too obvious to require 
elaboration. 
To compare variability on an abstract basis an expression 
combining the idea both of standard deviation and type is added 
here. It is found by dividing the standard deviation by the 
mean as a base. The result is an excellent index of variability 
in the form of a rate percent usually known as the coefficient 
of variability. The value of the coefficient of variation will 
change directly with changes of the standard deviation, and 
inversely with changes of the mean. For the case at hand the 
coefficient of variability is 54.60 and 33.28 for the bog con¬ 
ditions and the woodlots respectively. 
The mode and the three important variations constants, 
together with the probable errors of the determination, which 
were deduced from the frequency curves in the manner described 
above, are as follows: 
Habitat. 
Bog 
Woodlots 
Difference 
Mode. 
3 mm. 
2 mm. 
1 mm. 
Mean. 
3.425±0.098 
1.701 ±0.038 
1.72 mm 
Standard Deviation. 
1.870 ±0.069 
0.566 ±0.027 
1.31 mm. 
Coefficient of Variability.. . 
54.60 + 2.55 
33.28 + 2.46 
21.32 
The amount of variation is, as we should expect it to be, 
sensibly different in each of the localities selected. The ex- 
treme values for the coefficients are 54.60 and 33.28 giving 
a difference of 21.32. We may accept these differences in the 
coefficients of variability as additional proof that when organ¬ 
isms are introduced in changed or unusual conditions they be¬ 
come more or less variable. It can safely be granted that the 
conditions of variability which are here a function of place, are 
masked but little by others. In the case at hand, variability 
is not due to chance but is an inevitable accompaniment of the 
differences in the habitat. The evidence for this statement is 
found especially in a forthcoming paper on the response of plants 
to toxic bodies, and in the methods and results of experimental 
physiology (2). Here, however, the results appear to be of 
