G. Udny Yule. 
PROFESSOR JOHANNSEN’S EXPERIMENTS 
IN HEREDITY: A Review. 
By G. Udny Yule. 
rpHE statistical theory of heredity, as developed in the work of 
Galton and Pearson, concerns itself with aggregates or groups 
of the population and not with single individuals. A number of laws 
have been formulated, on the basis of researches conducted in 
accordance with such theory, of which the most important is Gabon’s 
Law of Regression, i.e. the law that the offspring of abnormal parents 
are on the average less abnormal than such parents, “ regressing ” 
towards the mean of the race. 
But the student of heredity, says Professor Johannsen 1 (p. 4), 
“ cannot rest satisfied with such essentially statistical researches. 
A race, a population, an aggregate (Bestand) of any sort ... .is 
by no means always, from the point of view of the biologist, to be 
treated as homogeneous (als Einheit), even when the individual 
variations are grouped round an average value, presumably “ typical,” 
in the manner expressed by the law of error. Such a population 
may.contain a number of independent types (selbstiindige 
Typen), differing markedly from one another, which may not be 
discoverable at all by direct observation of the empirical frequency 
curve or table.” 
“ Before such a population is treated as homogeneous ” he 
continues (p. 5) “it should therefore be biologically analysed in order 
to be clear as to its elements, that is to have some knowledge as to 
the independent types (selbstandige Typen) already existing in the 
population. Only after such an analysis is one in a position to 
decide whether, and how far, a treatment of the material as 
homogeneous is permissible. It is in fact such an analysis 
that Louis Leveque de Vilmorin had in mind when he.laid 
down his ‘ principle of isolation ’. According to this principle 
one must decide by isolation—that is by keeping separate the seeds 
of each individual mother plant—whether the ‘ force hereditaire,’ 
as Vilmorin terms it, be large or small.” 
1 Ueber Krblichkeit in Populationen und in reineu Linieu, ein 
Jieitrag zur Beleuelitung scliwebender Selektionsfragen : 
von W. Johannsen, Professor der Pflauzenpliysiologie an 
der kgl. danischeu landw. Iloclischule in Kopenliagen. 
P'isclier, Jena 1903 (pp. 68). 
