10 Berry — Present Tendencies in Paleontology. 



persal of animals and plants that have been going on 

 since the beginning of life — as well as the rapidity of 

 radiation of different types of organisms. Paleogeog- 

 raphy will be on a far less speculative footing when it 

 rests on proof and not on authority. 



I do not believe that we can safely generalize with our 

 present stock of accumulated knowledge. Take a theo- 

 retical case of a transgression and assume that the rate 

 of change of level amounted to a foot a century, which I 

 suppose would be considered fairly rapid, and that the 

 submergence amounted to 500 feet, the time involved 

 would be 50,000 years. The Upper Cretaceous trans- 

 gression represented by the Dakota sandstone and Ben- 

 ton involved perhaps twice as great a change of level, and 

 disregarding any halts or oscillations of the strand it 

 would still mean that 100,000 years were involved in the 

 operation. Inevitably there would be changes in salinity 

 and climate which must be reflected in the faunas. It 

 seems to me that we must either admit a certain measure 

 of validity of homotaxis in all except special cases or 

 assume that the breaks between faunally distinct forma- 

 tions represent very great lapses of time. On the other 

 hand changes in faunal facies in passing from a forma- 

 tion like the Onondaga to the Hamilton mean merely a 

 change in local environment such as is, I imagine, respon- 

 sible for most examples of recurrent faunas, so-called. 



The question is also influenced by what the term fauna 

 denotes. Does it mean the whole biota or only certain 

 forms considered as typical. Certainly I should expect 

 Belemnitella to spread more rapidly than the contem- 

 poraneous Exogyra, or an Echinoid more rapidly than a 

 Pentremite. The varying vitality or organisms under 

 adverse conditions, either as mature animals or in the 

 larval state, is also a factor of great importance. Larval 

 oysters are very intolerant under adversity while other 

 sedentary molluscs have a much more hardy progeny. 

 Another factor in distribution is the relative length of the 

 free swimming larval stage in sedentary forms. There 

 are wide limits of usage as to what characterizes a fauna 

 and what are its critical members. Shall we rely on its 

 more abundant dominant species, on the percentage of 

 species common to another fauna of known age, to the 

 first or the last appearance of certain forms, or shall we 

 place the greatest weight upon the rarer short lived 



