THE ORIGIN OF THE OLDEST FOSSILS, ETC. 477 



These emigrants, like the civilized men who invade the homes 

 of peaceful islanders, brought with them the improvements 

 which had come from fierce competition, and they have carried 

 everything before them and produced a great change in the 

 pelagic fauna. 



The rapid intellectual development which has taken place 

 among the mammals since the middle Tertiary, and the rapid 

 structural changes which took place in animals and plants when 

 the land fauna and flora were established, are well known, but 

 the fact that the discovery of the bottom initiated a much earlier 

 and probably more important era of rapid development in 

 the forms of animal life has never been pointed out. 



If this view is correct the primitive fauna of the bottom 

 must have had the following characteristics: I. It was entirely 

 animal, without plants, and it at first depended directly upon the 

 pelagic food supply. 



2. It was established around elevated areas in water deep 

 enough to be beyond the influence of the shore. 



3. The great groups of animals were rapidly established from 

 pelagic ancestors. 



4. The animals of the bottom rapidly increased in size and 

 hard parts were quickly acquired. 



5. The bottom fauna soon produced progressive develop- 

 ment among pelagic animals. 



6. After the establishment of the fauna of the bottom 

 elaboration and differentiation among the representatives of 

 each primitive type soon set in and led to the extinction of 

 connecting forms. 



Many of the oldest fossils like the pteropods are the modi- 

 fied descendants of ancestors with hard parts, and there is no 

 reason to suppose that the first animals which were capable of 

 preservation as fossils have been discovered, but it is interesting 

 to note that the oldest known fauna is an unmistakable approxi- 

 mation to the primitive fauna of the bottom. 



The lower Cambrian fossils are distributed through strata 

 more than two miles thick, some, at least, of them showing by 



