28 



Pedigrees 

 still selected 

 from 

 pedigrees. 



Absence of 

 hereditary - 

 variations in 

 pedigree 

 cultures. 



Theory of 

 unit- 

 characters. 



Johannsen's 



pure-line 



theory. 



With the introduction of the pedigree culture system there did not 

 follow immediately a rejection of the original principle of continuous selection. 

 The new system was regarded useful only as a means of obtaining in the 

 shortest possible time, constant and uniform sorts. The Darwinian idea of 

 the omnipresence of hereditary variation in all life was still held by Nilsson 

 who regarded it necessary to continue the selection from generation to genera- 

 tion in order to effect a complete fixation of the characters, while at the same 

 time he believed that continuous selection was still capable of effecting im- 

 provements even upon sorts already fixed (32, p. 13). This idea came to 

 be abandoned in due time when it was discovered, as we shall see later, that 

 the variations which were often noticed in these small plots were mere modi- 

 fications, induced by abnormalities in such external factors as soil, moisture, 

 etc., and that these were not hereditary. 



The appearance in 1900 of the views expressed by Mendel and DeVries, 

 together with those communicated in 1903 by Johannsen (14), the noted 

 Danish investigator, served to place the whole phenomenon of variation in an 

 entirely new light and seemed to explain at once, in a most convincing and 

 logical manner, the main circumstances upon which the occurrence of heredi- 

 tary variations depends. The principle involved in these views is that a 

 plant or animal is composed of distinct and independent Unit Characters, 

 units because they are capable of being treated #s such. These units were 

 regarded by Bateson as corresponding, in a sense, with atoms in chemistry. 

 While their nature is still a subject for speculation, this author (2 p. 266) 

 suggests that the operations of some units may be carried out by the formation 

 of definite substances acting as ferments. By the recombination of unit 

 characters through hybridization new ' compounds ' or combinations may be 

 effected which may appear and act quite differently. The characters entering 

 into such a combination, however, are not themselves affected, but may be 

 separated and recombined by future crossing to form other combinations 

 equally distinct in character. 



If this theory of the unit constitution of individuals be correct then 

 hereditary variations must obviously arise either directly as spontaneous 

 changes ("mutations") or as the result of the combination and subsequent 

 segregation of unit characters through hybridization. 



In his classical researches in connection with problems in heredity, 

 inspired as they were largely by the work of Vilmorin of Paris, Johannsen 

 showed the scientific necessity of working with what he termed pure-lines 

 when seeking to establish first principles. By a pure line he means the 

 progeny of a single, self-fertilizing individual. His investigations served 

 further to modify previous conceptions of heredity as expressed in Galton's 

 "Law of ancestral heredity." Galton (13) worked with "crowds" or popu- 

 lations of individuals and annunciated that the general type of a given crowd 

 can be changed or "shifted" by tbe selection of variations of a specific 

 character (plus or minus variants). Johannsen's investigations were con- 

 ducted with pure lines, that is the progeny of single individuals within a 

 crowd. The plants chosen were beans and barley, both of which are 

 normally self -fertilized and therefore easy to keep pure. The constituents 

 of all pure lines worked' with showed normal fluctuations which grouped 



