BIOMETRY 373 
parent is gifted by nature, the more rare will be his good fortune if he 
begets a son who is as richly endowed as himself.” This so-called law 
of filial regression is represented graphically in Figure 66 in which the 
actual stature of individual parents is shown by the oblique line, 
the stature of children by the dotted curve, and the mean stature of 
the race in the horizontal dotted line. 
Statistical vs. physiological methods.—One of the chief aims and 
results of statistical studies is to eliminate individual peculiarities and 
to obtain general and average results. Such work may be of great 
importance in the study of heredity, especially where questions of the 
occurrence or distribution of particular phenomena are concerned; 
but the causes of heredity are individual and physiological, and 
averages are of less value in finding the causes of such phenomena than 
is the intensive study of individual cases. 
By observation alone it is usually impossible to distinguish between 
inherited and environmental resemblances and differences, and yet 
this distinction is essential to any study of inheritance. If all- sorts 
of likenesses and unlikenesses are lumped together, whether inherited 
or not, our study of inheritance can only end in confusion. The 
value of statistics depends upon a proper classification of the things 
measured and enumerated, and if things which are not commensur- 
able are grouped together the results may be quite misleading and 
worthless. 
Statistical studies insufficient—Unfortunately Galton and Pear- 
son, as well as some of their followers, have not always carefully dis- 
tinguished between hereditary and environmental characters. Fur- 
thermore much of their material was drawn from a general population 
in which were many different families and lines not closely related 
genetically. Consequently their statistical studies are of little value 
in discovering the physiological principles or laws of heredity. Jen- 
nings (1910) well says, ‘‘Galton’s laws of regression and of ancestral 
inheritance are the product mainly of a lack of distinction between 
two absolutely diverse things, between non-inheritable fluctuations 
on the one hand, and permanent genotypic differentiations on the 
other.” In the case of man we have few certain tests to determine 
whether the differential cause of any character is hereditary or environ- 
mental, but in the case of animals and plants, where experiments may 
be performed on a large scale, it is possible to make such tests by (1) 
experiments in which the environment is kept as uniform as possible 
while the hereditary factors differ, and (2) experiments in which, in a 
