DISCRIMINATION OF TIME-VALUES IN GEOLOGY 577 



morphologic peculiarity by which it could be distinguished, 

 each such individual would represent a definite point of time, 

 and the succession of individuals thus distinguished would 

 express the history of the world in terms of the life periods of 

 the successive individuals. Practically it is impossible so to 

 distinguish the characters of individuals that they can serve as 

 time indicators. 



The naturalist does, however, recognize characters of suffi- 

 cient distinctness to differentiate one species from another. 

 Specific characters, like individual characters, have been acquired 

 gradually as generation has succeeded generation in ordinary 

 reproduction. The process of acquiring specific characters is 

 called evolution ; and specific evolution takes longer time than 

 individual development. The length of time during which in 

 any particular race generation continues without appreciable 

 disturbance of the specific characters of the offspring, varies in 

 different lines of descent, and specific characters are more 

 readily distinguished in one case than in another. But in all 

 cases the continuity of the specific characters without special 

 change represents a considerable period of geological time, and 

 this length of time is measured by the presence in the formations 

 of the same species of fossils. The period of the continuance 

 of the same species is therefore a definite length of time for 

 each species, since each species began at some definite time, and 

 (unless now living) became extinct at a later definite point of 

 time. This definiteness of the life-period of a species is inde- 

 pendent of our present knowledge either of its measure in years, 

 or of the thickness or stratigraphic position of the formation in 

 which the fossils are preserved. 



Having noted that the morphologic characters of organisms 

 are temporary in nature, we may further observe that this tem- 

 porary quality has to do with the vitality of the organism in a 

 measurable way. As in the case of two living organisms we 

 consider the vigor of that one to be the greater, which, under 

 similar circumstances, lives the longer; so, in general, the length 

 of endurance of a species may be taken as the measure of some 



