422 The American Naturalist. [May, 
ciples which seems to be preliminary to an intelligent collec- 
tion and arrangement of facts, upon the ground that a mere 
catalogue of facts will have no result. Variation is to be re- 
garded as one of the two modes or expressions of Heredity, or 
as the exponent of old hereditary forces developing under new 
or unstable conditions. It stands in contrast not with Hered- 
ity, which includes it, but with Repetition as the exponent of 
old forces developing under old or stable conditions. Nägeli 
ten years ago’ laid stress upon this, as have latterly Weismann, 
Bateson, Hurst,* and others. Nevertheless it is still widely 
misconceived. Hurst even regards Variation as the oldest 
phenomenon—an error in the other extreme, for they are 
rather coincident phenomena—representing the stability or 
instability -of development. The broadest analysis we can 
-make is that variations are divided by three planes—the plane 
of time, the plane of cause, and the plane of fitness. This raises 
the three problems to be solved regarding each variation ; 
when did the variation originate? what caused it to originate? 
is it or is it not adaptive? 
The student of heredity, in connection with these three 
planes of analysis, has then to consider the modes of heredity 
as complementary or interacting, for as soon as a ‘ variation’ 
recurs in several generations itis practically a ‘ repetition,’ and 
the repetition principle is a frequent source of apparent but not 
real variation or departure in the offspring from parental or 
race type. This relation becomes clear when we consider 
variations in man as seen in Anatomy and in Galton’s studies 
of inheritance and as expressed in the following table :— 
7“ Vererbung und Veränderung sind, wenn sie nach dem wahren Wesen der 
Organismen bestimmt werden, nur scheinbare Gegensätze.” Theorie der Abstam- 
mungslehre, p. 541 ; 
8 Biological Theories. I, The Nature of Heredity. Natural Science, vol. I, No. 
7, September, 1892. II, The Evolution of Heredity. Natural Science, vol. I, 
No, 8, October, 1892 
