THE HEREDITARY BASIS 15 
evident impress on much of recent thinking on the doctrine of 
evolution. 
The discovery which has meant most for the progress of ge- 
netics is unquestionably Mendel’s law. The product of years of 
research in the garden of the monastery at Briinn, Austria, the 
principles enunciated by Mendel, owing to the fact that they were 
published in a little-known journal, The Proceedings of the 
Natural History Society at Briinn, failed to attract the attention 
of the scientific world until they were made known independently 
by three investigators, Tschermak, Correns and De Vries in the 
year 1900. Thus began, with the beginning of the 2oth century, 
a new era in the study of genetics. Progress in this field since 1900 
has taken place at a very rapid rate. The amount of literature 
devoted to the subject suddenly swelled to several times its 
previous volume, and it is probably no exaggeration to say that 
since the rediscovery of Mendel’s law a greater advance has been 
made toward a scientific analysis of the phenomena of heredity 
than had been made during all preceding time. 
Mendel’s law embraces two principles designated commonly 
as (1) the law of dominance, and (2) the law of segregation. Ac- 
cording to the first, when two related but contrasted characters 
are brought together in a cross the one appears to the exclusion of 
the other. Mendel found, for instance, that when he crossed tall 
and dwarf peas the immediate progeny were all tall instead of 
intermediate in height. When he crossed green and yellow peas 
the first generation (called the first filial or Fi generation) con- 
sisted entirely of yellow peas. The characters tall and yellow 
are designated dominant in contrast to dwarf and green which 
are called recessive. 
The recessive characters are not lost, as is shown when the 
members of the Fi generation are either interbred or self-polli- 
nated. They appear in the second or F2 generation along with a 
certain proportion of dominants. Numerous experiments have 
shown that in typical cases the dominant and recessive characters 
are segregated in the second generation in the proportion of three 
dominant to one recessive. The separation of the original char- 
