CHAPTER XII 

 CONCLUSION 



1. Size Regulation and Other Regulations 



The facts which have passed in review in these chap- 

 ters are easily summarized qualitatively, by the 

 statement that practically every factor which has been 

 investigated has been found to modify body size in 

 some species or another of unicellular organisms. 

 Only quantitative results can be used to decide how 

 the individual organism "knows" when its proper size 

 has been attained. 



Why variability? Some facts indicate that size is 

 not wholly a question to be answered by the individual 

 but rather by the racial population. For instance, in 

 a certain population one per cent of the individuals 

 have double the mean body volume and another one 

 per cent have half the mean volume. Such a statistical 

 outcome might be regarded as evidence of a factor 

 beyond the control of the single member of the group. 



From another aspect the variations may be regarded 

 as due to differences in some physiological function 

 other than the rate of building up of body substance. 

 Perhaps size is controlled very accurately by a pattern 

 of functions, while the functions in turn are the pri- 

 mary variants. This also leads at first to an apparent 

 impasse, for then these constituent functions have 

 varied in spite of the genetical and environmental uni- 

 formity. 



Or, shall it be supposed that the inheritances and 

 the conditions are not uniform? With respect to in- 

 heritances it is clear that any one of the variant indivi- 

 duals can be the progenitor of a population whose 



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