THE ORIGIN OF THE OLDEST FOSSILS, ETC. 289 



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, de- 

 pended 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 development 

 among pelagic animals. 



6. — After the establishment of the fauna of the bottom, elabo- 

 ration and differentiation among the representatives of each primi- 

 tive type soon set in and led to the extinction of connecting forms. 



Many of the oldest fossils, like the pteropods, are the modified 

 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 approximation 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 their fine 

 grain and by the perfect preservation of tracks and burrows which 

 were made in soft mud, and of soft animals like jelly-fish, that they 

 were deposited in water of considerable depth. The sediment 

 was laid down slowly and gently in water so deep as to be free from 

 disturbance, and under conditions so favourable that it contains 

 the remains of delicate animals not often found as fossils. 



While the fauna of the lower Cambrian undoubtedly lived in 

 water of very considerable depth, it was not oceanic, but continen- 



