II PROGRESS OF PALAEONTOLOGY 43 



tion ; if the stratification of the rocks is not the 

 effect of such causes as at present produce stratifi- 

 cation, we have no means of judging of the dura- 

 tion of past time, or of the order in which the 

 forms of life have succeeded one another. But if 

 these two propositions are granted, there is no 

 escape, as it appears to me, from three very 

 important conclusions. The first is that living 

 matter has existed upon the earth for a vast length 

 of time, certainly for millions of years. The 

 second is that, during this lapse of time, the forms 

 of living matter have undergone repeated changes, 

 the effect of which has been that the animal and 

 vegetable population, at any period of the earth's 

 history, contains certain species which did not exist 

 at some antecedent period, and others which ceased 

 to exist at some subsequent period. The third is 

 that, in the case of many groups of mammals 

 and some of reptiles, in which one type can be 

 followed through a considerable extent of geological 

 time, the series of different forms by which the type 

 is represented, at successive intervals of this time, 

 is exactly such as it would be, if they had been 

 produced by the gradual modification of the 

 earliest forms of the series. These are facts of the 

 history of the earth guaranteed by as good evidence 

 as any facts in civil history. 



Hitherto I have kept carefully clear of all the 

 hypotheses to which men have at various times 

 endeavoured to fit the facts of palaeontology, or by 



