The two great breaks in the history of life

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Abstract
Summary: Very early in the history of geology it became recognized that past time, so far as the history of life was concerned, fell into three periods: Primary, Secondary and Tertiary. Each contained fossils, easily recognized—trilobites, ammonites and mammals—which as groups characterized the periods, coming in with them, dying out at their ends. When evolution became the accepted explanation of the successive series of animals and plants the existence of “ gaps ” between the faunas of the three periods implied evolutionary events not easily understood, and their nature and significance have often been discussed. Most of such discussions have been based on the history of marine invertebrates, and are inconclusive. Thus some study of the events through their effects on land-living vertebrates throws further light on the phenomenon and its evolutionary significance.

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