212 LYELL'S ELEMENTS OF GEOLOGY. 



Absence of Mammalia in the Wealden. 



terns.* They may have been contemporaneous deltas of other 

 rivers flowing from the same ancient continent. 

 ~- Absence of mammalia. Among the numerous fossils of the 

 Wealden, no remains of mammalia have been hitherto detected , 

 whereas we should naturally expect, on examining the deposits 

 recently formed at the mouths of the Quorra, Indus, or Ganges, 

 to find, not only the bones of birds, and of amphibious and land 

 reptiles, but also those of such warm-blooded quadrupeds as fre- 

 quent the banks of rivers, or, like the hippopotamus, inhabit 

 their waters. Would not the same current of water which drifted 

 down and rolled the bones of the lizards, tortoises, and fish of 

 the Wealden, have also swept down into the delta some frag- 

 ments at least of mammiferous bones, had any animals of the 

 highest class been then in existence ? As a general rule, indeed, 

 we cannot lay much stress on mere negative evidence ; and it 

 may be well to notice, that although so many teeth of the Iguan- 

 odon have been collected, it is only of late that a single small 

 portion of a jaw of one of these gigantic lizards was obtained. 

 Perhaps, in like manner, some bone or tooth of a fossil quadru- 

 ped will one day be found. We may at least say, that we have 

 at present no example of a continent covered with a luxuriant 

 vegetation, and forests inhabited by large saurians, both aquatic 

 and terrestrial, and by birds, yet at the same time entirely desti- 

 tute of warm-blooded quadrupeds. The nearest analogy to this 

 state of things is that of New Zealand ; and this fact will be 

 more particularly alluded to in the sequel. (See p. 256.) 



In conclusion I may remark, that from the time of the com- 

 mencement of the Wealden, to far on in the cretaceous period, 

 we have signs of subsidence, and consequent diminution of land. 

 But after the chalk was formed, or during the tertiary periods, 

 we have, on the contrary, proofs of an increase of land in 

 Europe. But we must not extend these generalizations to the 

 whole surface of the globe ; for other large areas may have been 

 growing more and more continental during the cretaceous, and 

 more and more oceanic during the tertiary periods, the direction 

 of the prevailing subterranean movement being reversed. 



* See Fitton, Geol. Trans., Second Series, vol. iv. p. 328., and his references. 



