x.] GEOLOGICAL CONTEMPORANEITY. 207 



A ideological formation is the sum of all the strata 

 deposited over the whole surface of the earth during 

 one of these epochs : a geological fauna or flora is the 

 sum of all the species of animals or plants which 

 occupied the whole surface of the globe, during one 

 of these epochs. 



The population of the earth's surface Avas at first 

 very similar in all parts, and only from the middle of 

 the Tertiary epoch onwards, began to show a distinct 

 distribution in zones. 



The constitution of the original population, as well 

 as the numerical proportions of its members, indicates 

 a warmer and, on the whole, somewhat tropical climate, 

 which remained tolerably equable throughout tke year. 

 The subsequent distribution of living beings in zones 

 is the result of a gradual lowering of the general 

 temperature, which first began to be felt at the 

 poles. 



It is not now proposed to inquire whether these 

 doctrines are true or false ; but to direct your atten- 

 tion to a much simpler though very essential preliminary 

 question — What is their logical basis ? what are the 

 fundamental assumptions upon which they all logically 

 depend ? and what is the evidence on which those 

 fundamental propositions demand our assent ? 



These assumptions are two : the first, that the com- 

 mencement of the geological record is coeval with the 

 commencement of life on the globe ; the second, that 

 geological contemporaneity is the same thing as chrono- 

 logical synchrony. Without the first of these assump- 

 tions there would of course be no ground for any 

 statement respecting the commencement of life ; with- 

 out the second, all the other statements cited, every 

 one of which implies a knowledge of the state of 



