220 GENERAL BIOLOGY 



Effect of Selection in Heredity. - - The fluctuating 

 variations which group themselves in a probability 

 curve have been especially noticed in domesticated 

 plants and animals, although they are probably 

 no more or less numerous in the wild types. In the 

 former, man has been long accustomed to pick out 

 or " select " the particular type that he favored for 

 breeding purpose, knowing that the progeny will be 

 similar. Thus, it is desirable in the culture of 

 sugar-beets to secure as high a percentage of 

 sugar as possible. The amount of sugar which the 

 beet-root stores up, although modified by external 

 conditions, such as sunshine or its lack, is due in 

 great part to the inherent qualities of the seed, 

 that is, is an hereditary character. In unselected 

 stock the percentage of sugar is usually 7 per cent to 

 14 per cent. If only those beets with the highest 

 percentage of sugar be taken for seed, it is possible 

 to increase this average in two to three years (that 

 is, to shift the mode of the curve toward the maximum 

 extreme), until a percentage of about 20 per ceni is 

 attained. But, curiously enough, further selection 

 will not increase that percentage, and moreover 

 constant selection is necessary in order thai tins 

 maximum be maintained. Such an experiment 

 shows, as did Galton's calculations, that the bulk 

 of inheritance is derived from the immediate parents 

 (otherwise selection would not alter the mode so 

 rapidly), and, secondly, that there is a definite limit 

 beyond which selection cannot alter the degree of 

 inheritance, or perhaps, we may say, beyond which 



