SO 
most essential points of his method, without some know- 
ledge of which it would be impossible to understand 
clearly what I am going to say about the Logarithmic 
Curve, which will be treated as a special case of these 
Skew Curves. 
Every individual has grown up under the influence of 
a complex of ,causes“, among which every amount of 
food, water, light, is to be reckoned. This complex has 
for each individual its own value; we will indicate it by 
the variable quantity z. 
The distribution of these z's is supposed to follow a 
normal curve. If, notwithstanding this, the distribution of 
the measures x attained by the individuals, is found skew 
and not normal, this must be due to the special manner 
in which the organisms have undergone the influence of 
these causes, or reacted upon them. 
When there is no hereditary diversity among the indi- 
viduals, the correlation between causes and measures will 
be complete. If the ones vary independently, the others 
may be said to vary dependently. Let us express their 
connection by the equation 
2— Er} 
Then, with an increase dz of the z, corresponds an 
increase dx of the x, so that 
dE (dx, or 
de LA 
(x) 
Here we see expressed the manner of reacting on the 
| : 
causes; the factor F(x) means that in general an increase 
of x during the period of growth depends on the value 
already attained. 
Kapteyn's method now consists in an endeavour to 
find a normal curve of z's related to the empirical skew 
curve of x's, sot hat equal frequencies exist between every 
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