504 GEOLOGY 



The early faunas of the Cambrian were somewhat provincial 

 in nature, but toward the close of the period, as the seas spread over 

 the continents, there was a marked tendency toward cosmopol- 

 itanism. The impression must not be gained that life was every- 

 where the same at a given stage. There was probably less varia- 

 tion geographically, during most geologic periods, than there is 

 to-day, though it is not certain that this was true of all past epochs. 

 It is improbable that there was ever uniformity over the whole 

 globe. 



The Succession of Faunas 



Under the doctrine of evolution, it is presumed that the life 

 of every past stage has grown out of that which immediately pre- 

 ceded it, and that it has merged into that which immediately 

 followed it. It is usually assumed that if no exceptional influences 

 affected the process, there was a continuous series of slow changes 

 without sharp lines of demarkation. If this conception were 

 realized in fact, it would be less appropriate to speak of a succession 

 of faunas than of one continuous ever-changing fauna. It is not 

 yet demonstrated, however, that evolution proceeded solely by 

 very slight changes coming in from generation to generation. It 

 may have proceeded by distinct and abrupt changes. 1 This doc- 

 trine, as now held, does not maintain that a whole fauna would be 

 likely to change into a different fauna abruptly, but merely that 

 new species might arise in it abruptly. Irrespective of this or any 

 other specific hypothesis, it is to be noted that the geological record, 

 as now known, does not show complete gradations from one species 

 into another. In some cases there is a close approximation to a 

 graded series from one species to another, but the steps of the 

 gradation are not sufficiently close and definite to decide between 

 evolution by an infinite number of small changes, and a smaller 

 number of greater changes. 



If we turn from species to faunas, it is obvious that a more 

 general point of view must be taken. Observation shows that in 

 some cases one fauna graduates into the succeeding one, while in 



1 DeVries Die Mutationstheorie, 1903. See also Bateson's Man-rials 

 for the Study of Variation, 1894; and W. B. Scott, Oil Variations and Muta- 

 tions, Am. Jour. Sci., 1894, p. 355. 



