LIFE 365 



is presumed that the life of every past stage has grown out of that 

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

 which immediately followed it. It is usually assumed that if no 

 exceptional inlluences came in, there was a continuous series of slow 

 changes without sharp lines of demarkation. If this conception 

 were reuli/ed in fact, it would be less appropriate to speak of a suc- 

 ccii>n of faunas than of one continuous ever-changing fauna. It 

 is not yet demonstrated, however, that evolution proceeded solely 

 I >v very slight changes coming in from generation to generation. It 

 may have proceeded by distinct and abrupt changes; 1 or at any 

 rate new species may have arisen abruptly, so far as now known. 

 Irrespective of any other specific hypothesis, it is to be noted that 

 the geological record, as now known, does not show complete grada- 

 tions from one species to another. In some cases there is something 

 of a graded series, 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, a more general point of view 

 must be taken. Observation shows that in some cases one fauna 

 grades into the succeeding one, while in other cases the change 

 appears to be abrupt. If the progress of life the world over could 

 be studied as a unit, it would probably appear that there was a 

 nearly perfect gradation of the life of one stage into that of the next. 

 This gradation probably was more rapid at some times than at others, 

 and it is quite certain that some forms changed more rapidly than 

 others. But when we limit our study to the succession of faunas on 

 any one continent, or to any one province, it is evident that the 

 progress of evolution in the region studied was interrupted by physi- 

 cal changes which affected the depth, temperature, or clarity of the 

 water, and the nature of the bottom, and that these changes brought 

 about variations in the character and distribution of life. There 

 set 111 to have been rather definite times of notable change, between 

 which faunas changed but slowly. Where the faunal change in a 

 conformable series is abrupt, and there is no evidence of a gap in the 

 record, the explanation is usually sought in the immigration of a new 

 fauna from some other region. 



In the study of faunal progress, therefore, there is occasion 



1 DeVries. Die Mutationstheorie, 1903. See also Bateson's Material for 

 tin Study of Variation, 1894; W. B. Scott, On Variations and Mutations, Am. 

 Jour. Sci., 1894, p. 355 ; and discussions of Mendel's theory. 



