294 H. S. WILLIAMS SHIFTING OF DEVONIAN FAUNAS 



It is a question of interpretation whether each particular phase of 

 expression of fluctuating characters is a matter of time or of environment. 



I have reached the conclusion that it is those species whose characters 

 have the greater degree of normal and persistent fluctuation which mi- 

 grate and follow the shifting conditions of environment, and their life 

 period is correspondingly longer. 



On the other hand, species whose plasticity of characters is narrow are 

 more closely adjusted to their environment, are local in their range of 

 habitat, and are temporary in their geologic life period. 



Interpreting the facts on this basis, it is the phases of continuously 

 fluctuating characters in species of wide geographic distribution and of 

 long geologic range which furnish the most satisfactory evidence of tem- 

 porary stages in the life history of faunas. 



Difficulties in establishing Evidence of strict Contemporaneity 



Another question of interpretation arises when we attempt to recon- 

 struct the physical condition of the environment at successive stages of 

 time. 



In a single vertical section we have positive evidence of succession in 

 time. If we were sure that no recurrence of the same fauna could take 

 place, we could correlate two vertical sections strictly upon the fauna 

 contained in the strata — on the hypothesis that the single fauna existed 

 but once, and when it ceased in a given section its whole life period had 

 been expressed. But the facts show us that this is not the case in nature. 

 In geological times, as in the present, we know that many distinct faunas 

 are living on the face of the earth at the same time, even for very similar 

 conditions of environment. It becomes, therefore, a very complex matter 

 to establish the fact oY contemporaneity or to correlate two sections in 

 which the order of faunas and the character of the sediments differ, which 

 is generally the case for any two sections separated by 50 miles from each 

 other, although on stratigraphic evidence they may be properly inter- 

 preted as covering the same interval of time. 



