Life Changes on the Globe. 329 



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 

 some 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 phy- 

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

 temporaneity 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 

 flora?, 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, judg- 

 ment 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 on the evening of another place. 



The extreme value so often assigned to negative evidence 

 has materially assisted spasmodic theories. It has led to the 

 improved 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 introduced, and it has also induced many autho- 

 rities to affirm in the most positive manner that the organized 

 beings of two epochs have been totally distinct. As an instance 

 of this common phraseology we may cite Professor M'Coy's 

 declaration that in Australia as in Europe "the greater part 

 of the country sank under the sea during the Tertiary period, 



