8 GENETICS IN RELATION TO AGRICULTURE 
are almost invariably self-fertilized. They consequently give rise auto- 
matically to populations which are composed entirely of pure lines. 
The pure line theory, therefore, has tremendous practical significance. 
The mutation theory adds yet another conception to those which have 
already been stated, namely that of occasional mutability of germinal 
elements. It is, therefore, directly contradictory to the pure line theory 
in its fundamental postulate; but the very great infrequency with which 
changes occur in germinal elements saves the pure line theory from 
inutility. Here the important result has been to establish firmly the 
occurrence of occasional, definite, discontinuous changes in germinal 
substance in consequence of which new characters are added to the 
heritage of the race. Much of the variability in individual characters 
which is exhibited by plants and animals appears to have had its begin- 
ning in mutational changes in the germinal substance. The mutation 
theory, therefore, is another consequence of genetic investigations which 
has far-reaching practical consequences. 
Fruitful as have been the results of the method of experimental breed- 
ing in prosecuting genetic research, students and investigators should 
not delude themselves as to the nature of the knowledge which it has 
yielded. It cannot stand alone as a mode of investigation, for even the 
present illuminating conception as to the structure and operation of the 
hereditary mechanism has been almost as much the result of cytological 
as of breeding investigation. But taking this conception in its present 
form, tremendous as has been the advance of recent years, this sort of 
knowledge cannot represent the ultimate goal of genetic research. 
Mendelism has given us the plan of heredity—the more intimate and 
fundamental knowledge of the material which is employed in the elabora- 
tion of that plan remain the task of some other mode of research. 
The Method of Cytology.—The method of cytology in genetic re- 
search is concerned primarily with questions of cell mechanism. It may 
be said to be directed toward the solution of two distinct problems, first 
the behavior of the hereditary elements in somatogenesis, the building 
up of the body, and secondly in the determination of the nature and 
operation of the mechanism which distributes hereditary elements from 
parent to offspring. These are matters of fundamental importance in 
genetic enquiry; it is unfortunate that the methods of dealing with the 
problems here presented are necessarily static and so little under the 
control of the investigator. Nevertheless even with these handicaps, the 
contributions of cytology to genetic interpretation are by no means 
inconsiderable. The determination of the equivalent distribution of the 
hereditary elements in the cell divisions of somatogenesis and the prob- 
able fact that every ultimate cell in the body normally possesses all the 
hereditary elements of the initial fertilized egg-cell have been established 
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