No. 510] HEREDITY AND VARIATION 



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constantly. The individuals of any race vary much 

 among themselves, but these differences are matters of 

 growth and environment, and arc not inherited. What 

 is produced in reproduction depends on the fundamental 

 constitution of the race, not on the peculiarities of the 

 individual parent. The fundamental constitution of the 

 race is resistant to all sorts of influences ; it changes only 

 in excessively rare instances, and for unknown causes; 

 in a study of thousands of individuals of Paramecium, 

 through hundreds of generations, hardly a single case 

 of such change was observed. 12 Most differences be- 

 tween individuals are purely temporary and without sig- 

 nificance in inheritance ; the others are permanent diver- 

 sities between constant races. Systematic and continued 

 selection is without effect in a pure race, and in a mixture 

 of races its effect consists in isolating the existing races, 

 not in producing anything new. 



To give in brief an account of the general results of 

 extensive work, it is necessary to make definite state- 

 ments, and to omit conditions, exceptions and qualifica- 

 tions. This the reader is asked to remember; the details 

 may be found in the original papers. The results are 

 based on study and measurements of more than 10,000 

 individuals of Paramecium, kept under experimental con- 

 ditions for many generations. But science is essentially 

 incomplete and its results at any time are not final. The 

 author expects to make strenuous attempts to overthrow 

 the generality of some of the results set forth. 



papers: certain individuals of a race acquired a hereditary tendency to 

 showed it less strongly. 



