GEOLOGY. 245 



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, Prof. Huxley shows 

 that when the lias of England and that of Germany, 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. Tak- 

 ing for an illustration the computation of the late Daniel Sharps 

 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- 

 poraneity or synchronism were assumed upon such evidence, we 

 should fall into serious mistakes : 



" Now suppose that a million or two years hence, when Britain has 

 made another dip beneath the sea and has come up again, some 

 geologist applies this doctrine, in comparing the strata laid bare by 

 the upheaval of the bottom, say of St. George's Channel, with what 

 may then remain of the Suffolk Crag. Reasoning in the same way, 

 he will at once decide the Suffolk Crag and the St. George's Channel 

 beds to be contemporaneous, although we happen to know that a 

 vast period (even in the geological sense) of time, and physical 

 changes of almost unprecedented extent, separate the two. 



" But if it be a demonstrable fact that strata containing more than 

 sixty or seventy per cent, of species of Mollusca in common, and 

 comparatively close together, may yet be separated by an amount of 

 geological time sufficient to allow of some of the greatest physical 

 changes the world has seen, what becomes of that sort of contem- 

 poraneity, the sole evidence of which is a similarity of facies, or the 

 identity of half a dozen species, or of a good many genera ? 



"And yet there is no better evidence for the contemporaneity 

 assumed by all who adopt the hypotheses of universal faunae and 

 florae, of a universally uniform climate, and of a sensible cooling of 

 the globe during geological time." 



Looking fairly at the evidence before us, we can only come to the 

 conclusion that " a Devonian fauna and flora in the British Islands 

 may have been contemporaneous with Silurian life in North America, 

 and with a carboniferous fauna and flora in Africa." What was 

 the case we have as yet no means of knowing, and while grounds of 

 decision are wanting judgment should hold suspense. If we may 

 compare the successive changes in various parts of the globe to the 

 movements of a clock, we must not assume the starting-point of any 

 series of operations to have been identical. Each country, so to 

 speak, may have had its own clock, all the clocks going upon the 

 same principle, and to a large extent with the same order in their 

 motions, but the dawn marked by one may correspond in actual time 

 with the noon or the evening of another place. 



The extreme value so often assigned to negative evidence has 

 materially assisted spasmodic theories. It has led to the unproved 

 and improbable assumption that our very limited search for the 

 remains of older periods enables us to decide authoritatively the 

 proximate periods at which fish, reptiles, or mammals were intro- 



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