F. FLUCTUATION 

 LECTURE XXV 



GENERAL LAWS OF FLUCTUATION 



The principle of unit-characters and of ele- 

 mentary species leads at once to the recognition 

 of two kinds of variability. The changes of 

 wider amplitude consist of the acquisition of 

 new units, or the loss of already existing ones. 

 The lesser variations are due to the degree of 

 activity of the units themselves. 



Facts illustrative of these distinctions were 

 almost wholly lacking at the time of the first 

 publication of Darwin's theories. It was a bold 

 conception to point out the necessity for such 

 distinction on purely theoretical grounds. Of 

 course some sports were well known and fluc- 

 tuations were evident, but no exact analysis of 

 the details was possible, a fact that was of great 

 importance in the demonstration of the theory 

 of descent. The lack of more definite knowl- 

 edge upon this matter was keenly felt by Dar- 



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