BIOLOGICAL CHANGE IN GEOLOGICAL TIME. Ill 



Although the facts of palfeontology are so multifarious, so 

 varied, and in some cases so apparently inconsistent with each 

 other, and even seemingly contradictory, that we cannot assign 

 specific causes for them, the only conclusion that observation of 

 Nature, and Science, seem to warrant is that biological changes 

 with introductions of new and extinctions of old species are 

 not due to any suspension or supersession of the Keign of Law, 

 and that, therefore, however difficult it may be to explain the 

 cause of specific changes, they are all due to natural causes. 



In some cases, indeed, it does not seem difficult to suggest a 

 cause of extinction, as in the case of the great dinosaurs of the 

 Secondary epoch. These great creatures required much food, 

 which sometimes might not be easily procurable, and their 

 heavy and unwieldy bodies and very small brains would not 

 assist them in their search for sustenance. So also the great 

 mammals of the Pliocene and Pleistocene periods would be 

 severely handicapped by their great food and water require- 

 ments when seasons were unfavourable, or changes of level or 

 temperature altered the quantity or character for the worse of 

 the plants on which they fed. Changes in physical geography, 

 as Lyell long since pointed out, are capable of producing great 

 effects on the fiora and the fauna of a region. By the slight 

 subsidence of an extensive coastal plain it may be fiooded by 

 sea- water, and immense forests of trees and jungle plants 

 destroyed, by which great herds of animals may lose the food 

 on which alone they can thrive. Great swarms of locusts, 

 again, have the power of devastating a wide extent of country, 

 and so may deprive of food multitudes of small animals by 

 which large carnivora may lose their prey and so die of 

 starvation. 



In his great work, The Prmciples of Geology, Lyell gives an 

 interesting summary of the far-reaching effect of such an 

 apparently small and unimportant thing as the transportation 

 of a few polar bears by drift-ice to an island in northern seas 

 before the time of man, such as Iceland has seen since its 

 colonisation by Norwegians, who have been able to prevent the 

 mischief by exterminating the invaders. In the absence of 

 armed men and stronger carnivora, " the deer, foxes, seals, and 

 even birds," on which polar bears sometimes prey, " would be 

 soon thinned down. But this would be a part only, and 

 probably an insignificant .portion, of the aggregate amount of 

 change brought about by the new invader. The plants on 

 which the deer fed, being less consumed in consequence of the 

 lessened numbers of that herbivorous species, would soon 



