LIFE PERIODS. 169 



of America, or too conspicuous species like the sluggish pigeon 

 of the Mauritius and the giant runner of New Zealand*. 



The duration of man as a race of created life can only be put 

 in comparison with that of any other and earlier race of beings 

 by some class of evidence applicable to both. History, in a large 

 sense, belongs to both ; Geology is part of primaeval history, 

 though Chronology can only be stated for a few of the numerous 

 families of man f. 



If we place in the order of their successive existence on the 

 earth, the several characteristic groups of life which Palaeonto- 

 logy has revealed, we shall construct a scale, which though not 

 indeed a chronology of the earth, is the first step toward such a 

 scale of time. It represents Life Periods, which if they cannot 

 now be referred to astronomical vicissitudes, are nevertheless so 

 related to them and dependent upon them, that to find common 

 measures of both is not an inconceivable, though it may be an 

 impracticable problem ; it is not obviously impossible, though it 

 may be really attended with insuperable difficulty. Even as the 

 motions of the once ' fixed ' stars begin to be traced into system, 

 and their almost inexpressible distance to be measurable from 

 the opposite parts of the earth's orbit ; so the ancient phenomena 

 of terrestrial life acquire consistent positions, and stand in intel- 

 ligible relations to the duration of man. 



Many such scales of ' Life Periods ' may be constructed, ac- 

 cording as the phenomena are more or less analysed ; but they 

 must all express the same general truths. The following may 

 be an example, and we pray the reader who dislikes hard names, 

 to excuse in this case what can seldom be avoided in advancing 

 branches of knowledge, the use of new terms -to express new 

 relations of thought. To modern geologists, terms and ideas of 

 this order are quite familiar. 



* See monograph on Dodo, by Strickland; and papers on Moa, by 

 Mantell and Owen (Phil. Trans. &c.). 

 f See Kenrick on Primaeval History. 



