THE PROGRESSION OF LIFE ON EARTH 



remarkable, therefore, that only a few chapters of the past 

 history of life have been clearly read and that a fact of gen- 

 eral significance may be known by not more than a single 

 observation. Nearly every fresh exploration adds something 

 new and shows how much depends on local conditions. So 

 far as it goes, all the evidence points in the same direction — 

 to the slow and regular advance of the tvorld of life in the 

 way already stated. No conflicting evidence has thus far 

 been discovered. 



The beginnings of life will probably never be known, for 

 there is reason to believe that the earliest animals were soft- 

 bodied, without skeletons. They probably originated in the 

 open sea and acquired hard parts only when they settled 

 down within reach of the surf. By the time that any of them 

 had gained enough skeleton to be regularly fossilized, toward 

 the dawn of the Cambrian period, members of most of their 

 early predecessors had disappeared, so that their earliest 

 history is unknown. Swarms of other soft-bodied animals 

 were living at that time, for more or less vague impres- 

 sions of them occur in a peculiar bed of greasy shale of 

 the Cambrian period in the Rocky Mountains of Canada. 



It is clear, however, that before backboned animals 

 appeared or before animals acquired skeletons, the backbone- 

 less groups flourished widely and were at some times and 

 places represented by larger animals than any of their kind 

 of later date. Great armoured cuttlefishes, for example, 

 and gigantic lobster-shaped animals were the rulers of the 

 seas before the earliest backboned animals — the fishes — 

 began to flourish. Soon after the appearance of fishes the 

 lower groups just mentioned lost their leading place, and 

 most of them died out. A new era had begun, in which 

 fishes increased both in numbers and in size. The Old Red 

 Sandstone, both of Europe and of North America, laid down 



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