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THE AMERICAN NATURALIST [Vol. XL VI 



most diminutive saccate form, 1 was surprised to find 

 that the campanulates present in the culture degenerated, 

 pari passu, with the other type. They became much 



of their outward serialization. Vet, generation after 

 generation, they maintained their cannibalistic habits, 

 their heavy musculature, and above all the eampanulate 

 type of trophi, the only change in these latter organs 

 being a reduction in size. 



In the main, then, transitional periods are brief ; transi- 

 tional forms few. Unfavorable conditions prolong some- 

 what the existence of both. But the species always is 

 soon eliminated or sets up a new equilibrium under the 

 new conditions. 



A few words further may be added at this point upon 

 the matter of fluctuating variation shown by the different 

 forms of this species. Without recording such varia- 

 tions mathematically, I have endeavored to ascertain as 

 fully as possible the answers to three questions: First, 

 how great is the amount of such variation? Second, is 

 fluctuating variation especially correlated with one or 

 other of the types of heterogenesis above described? 

 And third, what causes are operative in producing it ? 



As to the amount of fluctuating variation, certain facts 

 have already been mentioned that come under this head- 

 ing. I will but add here the general statement that each 

 of the three types is, in itself, highly variable— quite suffi- 

 ciently so to be regarded as a decidedly variable species 

 were it really an independent form. 



As to the second point, the question of the correlation 

 between fluctuating variation and the mutation-like 

 transitions, this has also been partially discussed under 

 the heading of transitional types. But it is necessary to 

 add the unqualified statement that no evidence has been 

 discovered for such correlation. Variation is one thing; 

 heterogenesis another. The two phenomena contrast, 

 rather than are related. Thus the transition from the 

 saccate to the humped rotifer is often made when the 



