8 GEOLOGICAL BIOLOGY. 



history of the development of the individual organism, as 

 artificial time-measures the clock or watch, or the regular 

 periods of day and night, satisfy the demand. When longer 

 periods are recorded, the seasons and years, with their arti- 

 ficial names, are sufficiently definitive. Human history deals 

 with still longer periods, marked by great events in the na- 

 tions : the rise or fall of a dynasty, the founding of a city, the 

 discovery of a continent, the living of some man of powerful 

 influence — these constitute landmarks by which to measure 

 the order of lesser events. These, as chronological measures, 

 are now easily applied, but in our studies in natural history we 

 soon pass beyond the reach of even such records. A very 

 few centuries back and human history ceases altogether; 

 therefore the time-scale for the history of organisms must rest 

 upon an entirely different kind of evidence. Another reason 

 renders the ordinary units of time useless for the study of the 

 history of organisms. The animals and plants associated with 

 the earliest known traces of man present only the most insig- 

 nificant amount of divergence from their living representa- 

 tives. In most cases the differences are not greater than 

 differences presented by the known descendants of common 

 ancestors within the memory of a single generation of men. 

 The period of human existence, however long or short that 

 may be, is too brief to record any but the more minute details 

 of those modifications of which paleontology teaches. It is 

 unnecessary to state that the records we are to study are 

 buried in the rocks. Everybody knows that the rocks must 

 be of considerable antiquity ; but when we pass beyond the 

 age of man, as an inhabitant of the earth, our ideas of time- 

 relations are necessarily vague ; even for scientific men these 

 time-relations, both their actual length, in terms of human 

 standard, and also their relative periods, are not matters of 

 simple arithmetical calculation. 



Theories regarding the Length of Geological Time. — The the- 

 ories underlying the interpretation of the rocks are far more 

 important than at first would appear. The common notion, 

 up to a very few centuries, and in some quarters a few 

 decades ago, was that the antiquity of the inhabitants, and 

 the world itself, did not exceed six thousand years. We now 



