296 GEOLOGY. 



invaders. Comparatively slight changes in the relations of sea to 

 land were competent to introduce or to remove barriers and to per- 

 mit such migratory movements (see pp. 668-672, Vol. I). The whole 

 process is closely analogous to the well-known succession of the human 

 races that has been brought about by the migrations of man in the 

 Old and New Worlds. 



Pondering upon these agencies, it becomes clear that in the study 

 of faunal progress there is occasion to recognize (1) rather abrupt 

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

 changes brought about by the more gradual inflow of foreign species, 

 and the gradual commingling of the immigrants with the resident 

 species; (3) very gradual changes, or nearly constant states, due to 

 the slow evolution of resident species when not much affected by immi- 

 gration or by physical changes; and (4) more rapid evolution due to 

 profound changes in the physical conditions or in other agencies affect- 

 ing the life, including perhaps the unknown causes that may have 

 brought about a mutating stage simultaneously in large numbers of 

 the leading species. 



The greatest of the faunal changes are those that mark the close 

 of " Eras " and of the greater " Periods " (using these terms in their tech • 

 nical senses) ; but within the " Periods " there were less radical changes 

 that marked off the lesser successions of the faunas. Within the Cam- 

 brian period, three stages in the life progress are recognized and these 

 are made the ground for distinguishing the Lower (or Olenellus), the 

 Middle (or Paradoxides) , and the Upper (or Dikellocephalus) fanuas of 

 the period. It is to be understood that there were not a few species 

 which lived from one stage into the next and constituted bonds between 

 the faunas, and that the demarkation is not very distinct, but there 

 were leading forms that were confined to the individual stages and 

 characterized them and their faunas. 



The Lower Cambrian or Olenellus fauna. 1 — The most characteristic genus of 

 the lower fauna is the Olenellus, a finely developed trilobite of which three 

 types are illustrated herewith, Olenellus gilberti (Fig. 126, b), Holmia (Olenellus) 

 broggeri (Fig. 118, a), and Mesonacis (Olenellus) vermontana (Fig. 126, a). All 



1 See The Fauna of the Lower Cambrian or Olenellus Zone, by Chas. D. Walcott, 

 Tenth Ann. Rept., U. S. Geol. Surv., 1888-89, and the papers therein referred to. 

 Mr. G. F. Matthew, who has given much study to the Cambrian faunas of the Canar 

 dian provinces, interprets the succession somewhat differently from Walcott, who is 

 here followed. See Matthew's papers referred to in foot-note, p. 280. 



