282 CLARKE— THE AGE OF THE EARTH. 



horizontal and vertical distribution of a fauna, whose time is a 

 Moment. 



The whole interpretation of these conceptions centers upon the 

 origin and endurance of a mutation, which in the proper paleontologi- 

 cal sense is a departure from a recognized species toward and into a 

 unit which, by determinate action of the genes producing variation, 

 will become another species. That is to say, the mutation is a clearly 

 recognizable entity in paleontology, is the bridge crossing from spe- 

 cies to species, the connecting link which establishes the continuity of 

 the chain. Apart from considerations of physiology only, the pale- 

 ontologist sees no further occasion for debating the existence of con- 

 necting links or of passages from species to species, or as to how 

 species originate. The mutation is the departure from the one, seek- 

 ing adjustment and failing, or seeking and finding it in what must be 

 recognized from accepted standards as a distinct specific form, a dif- 

 ferent species from its parentage. But when it comes to a matter of 

 determining the rates, the time measure of these changes under vary- 

 ing and all conceivable physical conditions, the pursuit seems to us 

 hopeless, hopeless a priori, hopeless in observation. There are species 

 that have held their own without change through the ages — " immortal 

 types " they have been called ; and there are others which have yielded 

 so rapidly to change that their evolution is explosive. The same 

 facts are true of groups of animals ; and for the entire organic world 

 there have been earth-wide periods of long stagnation as well as of 

 rapid intensive change. So long as an estimate of the age of the 

 earth rests on evidence of the rate of change or adjustment in organ- 

 isms through the acquisition of new characters, we may as well 

 abandon the attempt to express it in concrete terms and satisfy our- 

 selves that for the development of life the duration of that fraction 

 of the earth's history is beyond human expression. 



