328 Life Changes on the Globe. 



" 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 the 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 attention 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 pro- 

 positions demand our assent ? 



" These assumptions are two : the first, that the commencement 

 of [that part of] the geological record [which has hitherto been de- 

 ciphered] is coeval with the commencement of life on the globe ; 

 the second, that geological contemporaneity is the same thing as 

 as chronological synchrony. "Without the first of these assumptions 

 there would of course be no ground for any statement respecting 

 the commencement of life ; without the second, all the other state- 

 ments cited, every one of which implies a knowledge of the state of 

 different parts of the earth at one and the same time, will be no less 

 devoid of demonstration. 



" The first assumption obviously rests entirely on negative 

 evidence. This is, of course, the only evidence that ever can be 

 available to prove the commencement of any series of phenomena : 

 but, at .the same time, it must be recollected that the value of nega- 

 tive evidence depends entirely on the amount of positive corrobora- 

 tion it receives. If A B wishes to prove an alibi, it is of no use 

 for him to get a thousand witnesses simply to swear that they did 

 not see him in such and such a place, unless the witnesses are pre- 

 pared to prove that they must have seen him had lie been there. 

 But the evidence that animal life commenced with the Lingrula-na°;s, 

 e.g. would seem to be exactly of this unsatisfactory uncorroborated 

 sort. The Cambrian witnesses simply swear they " haven't seen 

 anybody their way;" upon which the counsel for the other side 

 immediately puts in ten or twelve thousand feet of Devonian sand- 

 stones to make oath they never saw a fish or a mollusc, though all 

 the world knows there were plenty in their time." 



Having thus defined the nature of his inquiry, Professor 

 Huxley shows that when the lias of England and that of Ger- 

 many, or the cretaceous rocks of Britain or of India are said 

 to be " contemporaneous/'' the word is most loosely employed, 

 and that no evidence exists by which synchronism of formation 

 can be demonstrated in either case. Taking for an illustration the 

 computation of the late Daniel Sharpc that thirty or forty per 

 cent, of the known Silurian mollusca are common to both sides 

 of the Atlantic, and by way of allowance for undiscovered 

 specimens, assuming that sixty per cent, are common to the 

 North American and British Silurians, he avers that if contem- 



