CONTINUOUS VARIATION 



not account for correlations between parent and offspring so high 

 in value as those observed. It was also considered, in spite of Galton's 

 earlier leanings towards a particulate theory, that discontinuity in 

 segregation of the kind which the niendelian theory demanded was 

 incompatible with continuous variation in the phenotype. This 

 notion seems to have been shared by the early mendclian geneticists, 

 for, just as Pearson and his school took the view that mendelian 

 theory was limited in its application to such heritable variation as 

 was not continuous, De Vries argued that the mendelian principles 

 were of universal applicability because no continuous variation was 

 ever heritable. This error was the only noticeable point of agreement 

 between the two schools of thought. 



The basis was laid for the interpretation of continuous variation 

 on mendelian principles by Johannsen in Denmark and Nilsson-Ehle 

 in Sweden. Johannsen took as his experimental material the dwarf 

 bean (Phaseolus vulgaris) which shares with peas the property of 

 regular natural self-pollination. Thus any bean which is not descended 

 from a deliberate cross-pollination will be expected, on the basis of 

 simple mendelism, to be homozygous for all or nearly all its genes. Its 

 progeny, in their turn, will be to the same extent like their parent 

 and like one another. They, with their descendants, will constitute 

 what Johannsen called a pure line. 



Johannsen started in 1900 with 19 beans, from which he derived 

 19 pure lines. In 1901 he had 524 beans whose individual weights 

 he recorded in centigrams. These in turn yielded 5,494 beans in 

 1902, and again he determined their weights. The essential results 

 are shown in Table 4. If we compare the weights of the 5,494 beans 

 with those of their 524 mothers we obtain the upper of the two 

 tables. It is clear that the average weight of daughter beans is related 

 to that of the mother, though the differences amongst the daughters' 

 averages are smaller than those amongst the mothers. Thus the 

 mothers range from the 20-cg. to the 70-cg. class, but the daughters' 

 averages range only from 43 '8 cgs. to 56-0 cgs. 



The mothers can be subdivided further according to the pure lines 

 to which they belong. The lower of the two tables shows the average 

 weights of the daughters from the mothers in the various weight 

 classes of 9 out of the 19 lines. These 9 were the most informative, 

 but the remaining 10 lines tell the same story so far as they go. 



62 



