CHAPTER XXXIV 

 HEREDITY IN PURE LINES 



In the last chapter we have seen how Galton formulated his law 

 of fihal regression, which means that average parents tend to produce 

 average offspring, but that exceptional parents tend to produce off- 

 spring less exceptional than themselves, but nevertheless more excep- 

 tional than the average. 



In studying this law the Danish botanist Johanssen saw in it a 

 possibility of racial improvement through the instrumentahty of con- 

 trolled selection. He thought that by continually selecting the most 

 exceptional parents in each generation the degree of regression toward 

 the average might be lessened until a pure non-regressing strain might 

 be produced. 



In order to simphfy the experiment and obviate the complexities 

 inherent in intercrossing, he selected a self-fertilizing type, using the 

 bean Phaseolus. Taking a set of nineteen beans from several plants, 

 choosing the largest bean, the smallest bean, and seventeen inter- 

 mediate types, he planted these, expecting to find that the bean pro- 

 geny of the large bean would be mainly large, those of the small bean, 

 small. In this he was disappointed, for the bean progeny of the large 

 bean fluctuated about a mean, some being large, some small, but the 

 majority average. Similarly the bean progeny of the smallest bean 

 were of various types, mainly average. Each of these nineteen pure lines 

 had a mean of its own, and irrespective of which one was selected (the 

 largest, the smallest, or the average) the mean and distribution of the 

 progeny was practically the same. Thus it was concluded that within 

 a pure line selection had no effect in modifying the character of size of 

 beans. The reason for this rather unexpected result is not far to seek. 

 Johanssen was selecting on the basis of somatic variations, the fluctuat- 

 ing variations of Darwin, that are merely due to more or less favorable 

 growth conditions and are not represented in the germ plasm, i.e., are 

 not hereditary. Each germ cell in a pure Une is supposed to have the 

 same hereditary units, and, if that is so, selection would not be expected 

 to effect any modification that would persist. 



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