HEREDITY IN PURE LINE 217 



respects, as well as in Mem hers of any pure lines were 



remarkably uniform but there was also a fair amount of fluctuation 

 with respect to quantitative characters. Even such characters as the 

 shape and extent of color markings, weight, fertility, etc., showed con- 

 siderable individual variability, but none of these differences proved to 

 be hereditary. 



Jennings tried still another type of pure-line material, using organ- 

 isms that reproduce by binary fission. He worked with a number of 

 differing clones (pure lines produced asexually) of Paramecium, a com- 

 mon ciliate protozoan. Each selected Paramecium was isolated in a 

 separate culture vessel, where it was allowed to multiply for several 

 generations. Size differences and other measurable differences were 

 noted in the original isolated parent individuals, and those of all the 

 progeny were measured, plotted, and averaged. It was found that a 

 different mean, mode, and average size was characteristic for each pure 

 line. If the largest individual is then isolated from each culture and 

 bred for a number of generations, its progeny will be of the same aver- 

 age proportions as that of the stock from which the largest individual 

 had been chosen, for they are genotypically unchanged, though differ- 

 ing phenotypically. 



Exactly what is inherited? — All that is passed on from one genera- 

 tion to another is an organized mass of protoplasm, or in gametic re- 

 production two such organized masses — an egg and a sperm that unite 

 to form a zygote. Characters, as such, are not transmitted or passed 

 on. A zygote, the biologic heritage, has no eyes, no feet, no brain, no 

 instincts. All it possesses is a very definite nuclear and cytoplasmic 

 organization, which, under an appropriate environment, has the po- 

 tentiality of producing a new individual with characters whose expres- 

 sion may be more or less modified by the differences in the environ- 

 ment within the organism or outside of it. 



Specifically, let us state exactly what was inherited in Johannsen's 

 pure lines. Pure Line I differs in its hereditary potentialities from 

 Pure Line II in that, under the given conditions of the environment, it 

 produces on the average heavier beans than Pure Line II is able to 

 produce. If a particular bean chosen from Pure Line I and one from 

 Pure Line II were allowed to develop under exactly the same environ- 

 ment, the offspring of Pure Line I would be heavier than that of Pure 

 Line II. But if the environment of Pure Line II were better than that 

 of Pure Line I, it might readily produce larger beans that the latter. 

 But this environmentallv induced difference would not be inherited. 



