HISTORICAL, ACCOUNT OF EVOLUTION THEORY 39 
resulting from his computations of parental generations with those of 
offspring, he arrived at two laws of heredity: the law of filial regres- 
sion, and that of ancestral shares of inheritance. The essence of the 
first was that the offspring of exceptional parents tend to regress 
toward mediocrity in proportion to the degree of parental excep- 
tionalness. The second law was really explanatory of the first, for it 
was found that the offspring inherit not only from parents, but from 
the various grades of ancestors, and it was the pulldown of a miscel- 
laneous ancestry that made for regression toward mediocrity. It 
appeared that half of the hereditary influence could be assigned to 
parents, half of the remainder to grandparents, half of the remaining 
remainder to great-grandparents, and so on down the line. 
Kari Pearson, a pupil and follower of Galton, has carried the study 
of biometry to a more highly refined state. His attempt has been to 
apply to the study of evolution the precise quantitative methods which 
are used in physics and in chemistry. While much of Pearson’s work is 
far beyond the range of the average professional biologist today, it 
is extremely useful as a tool in handling data in which great accuracy 
is demanded. Frequently, however, the methods are far too refined 
for the material, and much time is wasted in handling crude data 
by means of highly refined instruments of measurement and ultra- 
accurate mathematical methods. 
On the whole the contributions of biometry to our understanding 
of the causes of evolution are rather disappointing. About the only 
clean-cut finding has been the discovery that some variations are 
continuous and others discontinuous. The former are capable of being 
expressed in a single curve with a single mode, while the latter are 
expressed in bimodal or polymodal curves. If material is homo- 
geneous to start with it is likely to give monomodal curves, but if it is 
heterogeneous, its heterogeneity will be revealed by the plural modes. 
In a subsequent connection (chapter xxv) some further account of the 
details of biometry will be presented. We must for the present be 
content with having placed biometry in its setting as one step in the 
advance of the evolution idea. 
MODERN EXPERIMENTAL EVOLUTION 
“While De Vries,” says Castle, “was engaged in his studies of the 
evening primrose he hit upon an idea far more important, as most 
biologists now believe, than the idea of mutation, though De Vries 
IW. E. Castle, Genetics and Eugenics (Harvard University Press, 1920), p. 82. 
