THE UNKNOWN FACTORS OF EVOLUTION. 83 



past research. There is little clear insight to be gained by 

 considering variations en masse, and in this lecture I shall put 

 forth some reasons why this is the case as well as some prin- 

 ciples which seem to be preliminary to an intelligent collection 

 and arrangement of facts, upon the ground that a mere cata- 

 logue of facts will have no result. Variation is to be regarded 

 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 Heredity, 

 which includes it, but with Repetition as the exponent of old 

 forces developing under old or stable conditions. Nageli ten 

 years ago 1 laid stress upon this, as have latterly Weismann, 

 Bateson, Hurst, 2 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 it is 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 expressed in the following table : 



1 " Vererbung und Veranderung sind, wenn sie nach dem wahren Wesen der 

 Organismen bestimmt werden, nur scheinbare Gegensatze." Theorie der Abstam- 

 mitngslehre, p. 541. 



2 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. 



