THE STATE GEOLOGIST. 43 



existence, weaker, less active, and less suitably adapted creatures 

 .giving way before their more successful competitors. It is a 

 general rule, also, that overspecialized forms, or those whose 

 habits and organization have responded so as to' conform to par- 

 ticular external conditions, are liable to perish when these condi- 

 tions change, through inability to readjust themselves in some 

 other direction. But it would appear, further, that races of ani- 

 mals have a life-period of their own, comparable to those of in- 

 dividuals, or the nations of mankind. Just as the history of the 

 latter resolves itself into periods of early development, dominant 

 culmination — or "Bliithezeit" as the Germans call it — and final 

 decadence; so species, genera and larger groups may be said to 

 pass through various stages of immaturity, maturity and senility. 

 Amongst old-age characteristics, whether of the individual or 

 the race, must be reckoned an increased incapacity for variation, 

 or decay of evolutional vigor; and after a certain point has been 

 reached, this road leads on to extermination, either sudden or 

 long-postponed. We shall have occasion to' observe presently 

 that there is a wide difference in longevity amongst various groups 

 of organisms. 



It will aid us to a graphic conception of the processes of evolu- 

 tion by likening them to a body rotating not always with uniform 

 velocity in an ascending spiral, and giving off particles which par- 

 take of its own motion. At irregular intervals the centrifugal 

 force is great enough to cause the particles to fly off in all direc- 

 tions, thus giving rise to what is known in palaeontology as "ex- 

 pression points." 1 Now these particles, which we may call 



1 The following definition of expression points is taken from Smith Wood- 

 ward's "Outlines of Vertebrate Palaeontology" (Introduction, p. xxi.) : 



"All known facts appear to suggest that the processes of evolution have not 

 operated in a gradual and uniform manner, but there has been a certain 

 amount of rhythm in the course. A dominant old race at the beginning of its 

 greatest vigor seems to give origin to a new type showing some fundamental 

 change; this advanced form then seems to be driven from all the areas where 

 the dominant ancestral race reigns supreme and evolution in the latter becomes 

 comparatively insignificant. Meanwhile the banished type has acquired great 

 developmental energy, and finally it spreads over every habitable region, re- 

 placing the effete race which originally produced it. Another 'expression 

 point' (to use Cope's apt term) is thus reached, and the phenomenon is re- 

 peated. The actinoptergian fishes furnish an interesting illustration. The 



