THE ORIGIN OF THE OLDEST FOSSILS. 369 



deejier waters or the sea tloor, and during this i^eriod the proper con- 

 ditions for the production of large and comi)licated organisms did not 

 exist, and even after the total amount of life had become very great it 

 must have consisted of organisms of small size and simple structure. 



Marine life is older than terrestrial life, and as all marine life has 

 shaped itself in relation to the pelagic food supply, this itself is the 

 only form of life which is independent, and it must therefore be the old- 

 est. Tliere must have been a long period in primeval times when there 

 was a pelagic fauna and flora rich beyond limit in individuals, but 

 made up of only a few simple types. During this time the pelagic 

 ancestors of all the great groups of animals were slowly evolved, as 

 well as other forms which have left no descendants. So long as life 

 was restricted to the surface no great or rapid advancement, through 

 the iniiuences which now modify species, was possible, and we know 

 of no other influences which might have replaced them. We are there- 

 fore forced to believe that the diilerentiation and improvement of the 

 primitive flora and fauna was slow, and that, for a vast period of time, 

 life consisted of an innumerable multitude of minute and simple pehigic 

 organisms. During the time which it took to form the thick beds of 

 older sedimentary rocks, the physical conditions of the ocean gradu- 

 ally took their present form, and during a part, at least, of tliis period 

 the total amount of life in the ocean nmy have been very nearly as great 

 as it is now without leaving any permanent record of its existence, for 

 no rapid advance took place until the advantages of life on the bottom 

 were discovered. 



We must not think of the populating of the bottom as a physical 

 problem, but as discovery and colonization, very much like the coloni- 

 zation of islands. Physical conditions for a long time made it impossi- 

 ble, but its initiation was the result of biological influences, and there 

 is no reason why its starting point should necessarily be the point where 

 the physical obstacles first disappeared. It is useless to si)eculate upon 

 the nature of the physical obstacles; there is reason to think one of 

 them, probably an important one, was the deficiency of oxygen in deep 

 water. 



Whatever their character may have been they were all, no doubt, of 

 such a nature that they lirst disappeared in the shallow water around 

 the coast, but it is not probable that bottom life was lirst established 

 in shallow water, or before the physical conditions had become favor- 

 able at considerable depths. 



The sediment near the shore is destructive to most surfiice animals, 

 and recent explorations have shown that a stratum of water of very 

 great thickness is necessary for the complete develojiment of the float- 

 ing microscopic fauna and flora, and it is a mistake to picture them as 

 confined to a thin surface stratum. Pelagic plants probably flourished 

 as far down as light penetrates, and pelagic animals are abundant at 

 very great depths. As the earliest bottom animals must have depended 

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