12 



toxodonts belong to the Eocene ; now these animals characterize the southern 

 continental life, or, as is the case with toxodonts, have but recently become 

 extinct there. This mode of defining those faunae is not, however, exact, 

 since many modern types have found their way into them, especially in the case 

 of Africa. 



How, then, is life significant of chronological station in the earth's strata? 

 Since very many forms of animals are so widely spread and at the same time 

 so distinctly limited in range on the earth's surface to-day, the same order 

 must have prevailed in past time, and have been of equal significance. That 

 this law of uniformity has prevailed in the past as in the present is amply 

 proven by the paleontology of a single zoological area taken by itself. The 

 apparition of types over the northern laiid-area has been nearly universal. 

 This fact has only been placed within our reach by modern investigations in 

 North America; for, until the sister continent of Europe-Asia was explored, 

 no one could be sure what degree of individual peculiarity her extinct life 

 might present. Now it is certain that the succession of Tertiary beds was 

 mutually similar, and that the contemporaneous deposits contained in a large 

 degree similar life, and that intermediate stages of the one can be properly 

 intercalated in the vacant interspaces of the other. The resemblances be- 

 tween the Lower Eocenes of New Mexico and Wyoming and that of France 

 are marked ; similarity between the Pliocenes of the respective continents is 

 evident. Descending in the scale, the parallels between the North American 

 and New Zealand Cretaceous are very apparent, and the faunae of the Caro- 

 linian and Wiirtembergian Trias were the same. The interruptions in the 

 record of life marked by the appearance of great land-areas near the close 

 of the Carboniferous and Cretaceous periods are universally observed in the 

 zoological areas of the northern hemisphere, or Arctogsea. The close of the 

 Cretaceous everywhere saw the end of ammonites, rudistes, and sauroptery- 

 gian and dinosaurian reptiles, in spite, in North America at least, of physical 

 continuity of deposits. 



Was this succession of interruptions of life universal over the globe, and 

 do these trenchant lines justify the old assumption of repeated destructions 

 and re-creations of animal life? The former question has already been 

 answered in the negative by the explanation of the characters of the existing 

 faunae of the southern hemisphere, where ancient types still remain in con- 

 siderable numbers. Moreover, some of the later periods of both North 



