THE CAMBRIAN PERIOD. 295 



for more extensive experimental studies to substantiate or overthrow 

 this most important doctrine. While awaiting the fate of this and 

 cognate biological doctrines, it is to be noted that the geological record, 

 as now known, does not show a complete gradation from one species 

 into another, though this may be due, as commonly assigned, to the 

 imperfection of the record. In some cases there is a close approxi- 

 mation 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 more abrupt 

 change, masked more or less by variations, as postulated by the muta- 

 tion theory. 



If we turn from species to faunas, which embrace a large number 

 of species belonging to quite different orders, it is obvious that a more 

 general point of view must be taken, and that inferences connected 

 with biological and physical environment, such as set forth in Volume I, 

 pp. 663-672, are to be brought under study. As a matter of obser- 

 vation, it appears that sometimes one fauna graduates into the suc- 

 ceeding one, while at other times the change is apparently 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 took 

 place more rapidly at some times than at others, and it is quite cer- 

 tain that some forms changed much more rapidly than others. But 

 when we limit our study to the succession of life of any one continent, 

 or to that of its coasts or embayments, or to that of some one of the 

 seas on its borders or on its bosom, or to that of any limited province, 

 it is evident that the progress of evolution in the given region was 

 subjected to interruption by physical changes, such as affected the 

 depth, temperature, or clarity of the water, the nature of the bottom 

 and like elements of the local environment, and that these brought 

 about shif tings in the distribution of life. Out of these local influences 

 superposed on the general progress of life there grew rather definite 

 stages of change between which the faunas retained a rather constant 

 and distinctive aspect, though always undergoing some modification. 

 Where the change is found to have been abrupt and there is no evidence 

 of a break in the record, the explanation is usually to be sought in 

 migration, by which a new fauna came in from somewhere else and 

 the old fauna emigrated to a new field, or was overwhelmed by the 



