2 MAINE AGRICULTURAL EXPERIMENT * STATION. I915. 



as units. It is clear that there is something in the germ cell 

 that is transmitted in its entirety and apparently unchanged 

 from one generation to the next. When organisms with two 

 opposing (allelomorphic) characters are crossed, each of these 

 characters is recovered in the second generation and each will 

 breed true in subsequent generations. Apparently such charac- 

 ters are essentially unchanged by having gone through the cross. 



Hence the present conception of the germ plasm is not that 

 of a plastic substance which can be moulded by the environment 

 or by selection but rather that of a mosaic made up of a vast 

 number of definite, stable units, each perfectly fitting into its 

 appropriate place. The hereditary process can be altered only 

 by changing or interchanging one or more of these units. In- 

 terchange of units can be easily affected by hybridization. How 

 experimentally to affect one of these units is one of the great 

 problems of modern biology. 



The ordinary iluctuating variations due to changes in environ- 

 ment in no way influence these independent units (factors or 

 genes) of the germ plasm and hence are not inherited. Only 

 changes which affect these germ plasm units can be transmitted 

 to the following generation. 



So much for the modern conception of the hereditary process. 

 It is clear that if this theory of inheritance is the true one, the 

 selection of fluctuating (somatic) variations will not influence 

 the characteristics of the offspring. 



In 1903 Johannsen announced that in self-fertilized plants 

 there was no eft"ect of selection within a "pure line." He de- 

 fined a "pure line" as the offspring of a single, self-fertilized, 

 homozygotic individual. In such a line all of the individuals 

 would possess exactly the same germinal constitution. Hence 

 except for the fluctuations caused by external conditions every 

 individual would be like every other individual. Johannsen sup- 

 ported his theory by a large amount of experimental evidence 

 from beans. 



This experimental result harmonized so well with the concep- 

 tion of the germ plasm outlined above and derived from othe*" 

 .sources that it readily gained credence among biologists. .Sinc^- 



^Johannsen, VV. Ueber Erblichkeit in Populationen und in reinen Linien. 

 Jena, 1903, pp. 68. 



