THE MUTATION THEORY 



559 



furnish the material for evolutionary changes. De Vries believed 

 this to be the case in E. lamarckiana and in other species, at least 

 at certain times in the history of the race. If his supposition were 

 correct the problem would be to discover the causes of the muta- 



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Fig. 308. — Fluctuations in pure lines of paramoecium. 



By rearing pure lines, or the descendants of single individuals in paramoecium, it was 

 found that each pure line fluctuated between certain limits. If all the individuals of this 

 figure were plotted in a single curve the result would be a curve of variation similar to that 

 shown in Fig. 307 (c/. Fig. 241). In the present figure, each row represents a single pure 

 line, showing the maximum, minimum, and intermediate sizes of the individuals. The 

 differences in size within the pure line are fluctuations due to differences in growth and 

 environment. The differences in average size between the several lines are hereditary and 

 comparable with mutations. 



The numbers show the length in microns. The average length for all the lines taken 

 together is given by the perpendicular line xx at 155 microns. The average size for each 

 pure line is that of the individual above which is placed a + sign. Other letters represent 

 investigator's special designations. (After Jennings, American Naturalist, 1909.) 



tions and the manner of their inheritance, together with any factors, 

 such as natural selection, that might determine the survival of 

 mutations once they had appeared. 



Mutations and Selection. — The relation of the Mutation 

 Theory to the Theory of Natural Selection will now be clear. If 



