MENDEL’S PRINCIPLES OF HEREDITY 
55 
and Rs breeding true, while the D(R)s split up in the propor- 
tion of 3 and 1 ; the recessive unit appearing again and 
again in each generation, and always in the same numerical 
proportion. 
The same facts have been deduced in theory and proved by 
experiment, when we have to deal with several contrasting 
characteristics in the same organism. 
Mendel’s papers deal only with the Pisum family (edible 
peas), and with the Hieracium (hawkweeds) ; he also mentions 
Phaseolus (French bean) and Lathyrus (sweet pea). It 
is to be noted that the doubtful results in Hieracium were 
probably due to the more recently discovered fact that this 
family is frequently parthenogenetic or apogynous, that 
is to say, the egg cells may develop without union with 
the pollen cells. 
Thomson will not admit that Mendelian phenomena are 
known in cases other than hybrids, but here he differs from 
other workers, and a considerable amount of evidence has 
already been accumulated to prove the occurrence of Mendelian 
principles quite apart from hybridisation. 
4 It will take many years before the far-reaching effects of 
Mendel’s law have been investigated, and it is more than likely 
that results of considerable importance may flow from its 
application in various novel directions (e.g. in the elucidation 
of the differentiation of sex, tentatively worked out by Berry 
Hart) ’ (Ency. Med.). 
* Mendel himself, indeed, admitted this possibility, and the 
work done since all tends to strengthen the view that sex 
inheritance follows lines very similar to those made familiar 
for other characters by experimental methods of breeding’ 
(C. J. Davies). 
Note again : ‘ The practical applications of Mendelian 
principles . . . will probably far exceed any limits we can yet 
perceive ; amongst them we can foresee not merely advances in 
the art of breeding animals and plants, but a control over the 
destiny of our own species ’ (Bateson). 
As to Sociology the same writer says : ‘ Genetic knowledge 
must certainly lead to new conceptions of justice, and it 
is by no means impossible that in the light of such, public 
