THE METHODS AND SCOPE OF GENETICS 7 
animals, for which reason it is sometimes called the pedigree method. 
The notable progress which has been made in genetics during the past 
few decades has come from the application of this mode of enquiry. It 
is the analytic method of the geneticist and it is often and not unjustly 
compared, both with respect to its utility and its limitations, to the test- 
tube method of analytical chemistry. From it have come many stimu- 
lating ideas of heredity and variation; the Mendelian theory of heredity; 
the closely related pure line theory of Johannsen; and the mutation 
theory of de Vries: few methods of research can boast a more honorable 
array of achievements. 
Of these achievements, the Mendelian theory is the accepted founda- 
tion of present ideas of heredity. Fortheapplication of Mendelian methods 
of analysis three essential breeding operations are necessary; first, the raising 
of pedigreed strains of plants and animals to determine their behavior 
under controlled conditions; second, the hybridization of diverse races; 
and third, the intensive study of the hybrid progeny through successive 
generations. From this outline of the breeding methods which are 
employed, it may be concluded rightly that the Mendelian method, like 
the Galtonian, is essentially statistical. It differs radically, however, 
from the Galtonian method in that it substitutes the observation of con- 
trolled progenies for that of ancestral generations. Its particular ad- 
vantage lies in the fact that it is strictly verifiable. Moreover, ithashada 
different and more specific purpose in view, namely to state in definite 
terms how the particular individual will behave in heredity, rather than 
to arrive at a determination of average behavior in this respect. The 
important result of this method of analysis has been to demonstrate that 
the germinal material is made up of definite units or factors which stand 
in close relationship to particular characters of the soma, and to demon- 
strate how these elements of the germinal substance are transmitted from 
generation to generation. 
The two remaining products of the pedigree culture method, the pure 
line theory and the mutation theory, stand in close relationship to the 
Mendelian theory of heredity; because they may be interpreted in terms 
of the elements which constitute the germinal substance. Of these the 
pure line theory may be said to add another conception to those of the 
Mendelian theory, namely that elements of the germinal substance 
possess a high degree of stability. If this conception be accepted, it 
follows—and this is the central postulate of the pure line theory—that 
variation among individuals of like germinal constitution is a response 
to external or internal conditions which are not reflected in the germinal 
substance. Such variations, therefore, are of no consequence for the 
establishment of new hereditary characters. A large number of plants, 
among them barley, oats, rice, wheat, and practically all the legumes, 
Digitized by Microsoft® 
