SUCCESSION OF LIFE UPON THE GLOBE. 387 



too, is a question of almost infinite dimensions, which can but 

 be glanced at here. Does Palaeontology teach us that the 

 almost innumerable kinds of animals and plants which we 

 known to have successively flourished upon the earth in past 

 times were produced separately and wholly independently of 

 each other, at successive periods ? or does it point to the 

 theory that a large number of these supposed distinct forms 

 have been in reality produced by the slow modification of a 

 comparatively small number of primitive types? Upon the 

 whole, it must be unhesitatingly replied that the evidence of 

 Palaeontology is in favor of the view that the succession of 

 life-forms upon the globe has been to a large extent regulated 

 by some orderly and constantly-acting law of modification and 

 evolution. Upon no other theory can we comprehend how 

 the fauna of any given formation is more closely related to 

 that of the formation next below in the series, and to that of 

 the formation next above, than to that of any other series of 

 deposits. Upon no other view can we comprehend why the 

 Post-Tertiary Mammals of South America should consist prin- 

 cipally of Edentates, Llamas, Tapirs, Peccaries, Platyrhine 

 Monkeys, and other forms now characterizing this continent ; 

 whilst those of Australia should be wholly referable to the 

 order of Marsupials. On no other view can we explain the 

 common occurrence of " intermediate " or " transitional " 

 forms of life, filling in the gaps between groups now widely 

 distinct. 



On the other hand, there are facts which point clearly to the 

 existence of some law other than that of evolution, and prob- 

 ably of a deeper and more far-reaching character. Upon no 

 theory of evolution can we find a satisfactory explanation for 

 the constant introduction throughout geological time of new 

 forms of life, which do not appear to have been preceded by 

 pre-existent allied types. The Graptolites and Trilobites have 

 no known predecessors, and leave no known successors. The 

 Insects appear suddenly in the Devonian, and the Arachnides 

 and Myriapods in the Carboniferous, under well-differentiated 

 and highly-specialized types. The Dibranchiate Cephalopods 

 appear with equal apparent suddenness in the older Mesozoic 

 deposits, and no known type of the Palaeozoic period can be 

 pointed to as a possible ancestor. The Hippuritida of the 

 Cretaceous burst into a varied life to all appearance almost 

 immediately after their first introduction into existence. The 

 wonderful Dicotyledonous flora of the Upper Cretaceous 



