No. 534] 



THE PURE LINE THEORY 



347 



II. The Fundamental Propositions of the Puee Line 

 Theory 



Our symposium has for its subject the Genotype or 

 Pure Line Theory. Some of the speakers have enthusi- 

 astically urged us to replace the words "pure line theory" 

 by "pure line facts." If this were done there would be 

 little need for this program. Pure line facts are as yet 

 a very insignificant part of biological data. The real 

 occasion for this symposium is the pure line theory — 

 the rank vines which have grown from the nineteen bean 

 seeds which Johannsen planted in 1901. Biologists 

 would have been little interested by the statement that 

 selection within the offspring of a single bean has been 

 ineffective in changing the weight of the seed. It is the 

 daring generalization of the conclusions drawn from 

 these limited experiments — the curt characterization of 

 other researches as of no biological sig-nilieanee or their 

 reinterpretation (from flounder's fins to intelligence in 

 school children) in terms of the bean experiments, that 

 forces us to take an interest in these matters. 



Our first problem is to ascertain what these generali- 

 zations — the elements of the pure line theory as con- 

 trasted with the pure line facts— are. Our second task 

 is to try to ascertain in how far experimental facts sup- 

 port the pure line theory. 



Davenport 2 has given a particularly good outline of 

 Johannsen 's theory: 



The fundamental principle of Johannsen is that an ordinary fre- 

 quency polvoon is usually made up «»t' measurements of a rliaracteristie 

 belonging to a non-homogenous mass of individuals; that it is really 

 mialy/.ahle into several elementary masses each of which has a "fre- 

 quency polygon" of its own. In each elementary polygon the varia- 

 tion is strictly due to non-inheritable somatic modifications, selection 

 of extremes of which has no genetic significance. But the selection for 

 breeding of individuals belonging to different elementary polygons, 

 lying, say, at the extremes of the complex, may quickly lead to an 

 isolation of these elementary polygons, the constituent individuals of 

 which reproduce their peculiarities as distinct elementary species. 



2 Davenport, C. B., Science, n. s., 30: 852, 1909. 



