THE CAMBRIAN PERIOD 505 



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 took place more rapidly 

 at some times than at others, and it is quite certain that some forms 

 changed much 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 physical 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. Out of these local influences 

 superposed on the general progress of life, there came to be rather 

 definite times of notable change, between which the faunas retained 

 a rather constant character, though always undergoing some modi- 

 fication. Where the faunal change in a conformable series is 

 abrupt, and there is no evidence of a hiatus in the record, the ex- 

 planation is usually to be sought in migration, as when a new fauna 

 came in from some other region, overwhelming the old fauna. 

 The whole process is closely analogous to the well-known succession 

 of human races brought about by the migrations of man. 



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

 to recognize (1) rather abrupt changes brought about by over- 

 whelming invasions; (2) less abrupt changes brought about by 

 the more gradual ingress of outside species, and the gradual com- 

 mingling of the immigrants with the resident species; (3) very- 

 gradual changes due to the slow evolution of resident species when 

 not much affected by immigration or by physical changes; and 

 (4) more rapid evolution due to profound changes in the physical 

 conditions or to other agencies less well understood. 



The abrupt appearance of the Cambrian fauna. The explana- 

 tion of the apparent suddenness of the appearance of the Cambrian 

 fauna is one of the open questions of geology. In a general way, 

 it may be said that older formations have been subjected to meta- 

 morphism, and that this tended to destroy their fossils; but this 

 suggestion is not altogether adequate, for some of the older forma- 

 tions are not greatly changed, and seem quite suitable for the pres- 



