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 general 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 world 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 softbodied, 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 impressions 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 backboneless 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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