§ 3. LAW OF EXTINCTION. 125 



2. Extinction of animals. — Before we enter upon the 

 geological history of the period immediately antecedent to 

 the present, it will be necessary to notice one of the most 

 remarkable facts which modern investigations have demon- 

 strated, namely, the annihilation of numerous species and 

 genera of animals and plants. 



From the continual transmutations in the distribution of 

 land and water, to which the earth's surface has been 

 subjected, as indicated by our previous remarks, the de- 

 struction of some forms of animal life would seem to be 

 a probable, if not inevitable, result ; and such proves to 

 have been the case. But not only has the extinction of 

 certain races taken place ; — a rigid examination of the 

 fossil remains entombed in the various strata, demonstrates 

 that other species, suited to peculiar states of the earth, 

 have successively appeared, and played their part in the 

 drama of life ; and when the physical conditions of the 

 lands and waters were altered, so as to be no longer 

 adapted for the continuance of those types of animated 

 nature, they in turn passed away, and were succeeded by 

 new forms of organization. 



The annihilation of whole tribes of animals and plants 

 has no doubt depended on a variety of causes ; and it is 

 probable that in the earlier ages, an important agent in the 

 extinction of many species, both terrestrial and aquatic, 

 was the frequent oscillation in the relative position of the 

 land and sea, and the consequent variations in atmospheric 

 conditions. 



3. Law of extinction. — But there is another consi- 

 deration that bears upon this problem, and to which I must 

 here briefly allude. It has been found that in countries 

 where certain formations follow each other in tranquil 

 and uninterrupted succession, the same restriction of par- 

 ticular fossils to special groups of strata, and the same dis- 

 appearance of some species, and the first advent of others, 



