THE CAMBRIAN PERIOD 335 



Cambrian life highly developed. The existence of life 

 in the earlier pre-Cambrian periods of the earth's history is 

 known only from the indirect evidence of organic sediments 

 and the like, or from the testimony of a few imperfect fossil 

 shells. In the Cambrian rocks, for the first time, we find 

 such shells abundant and varied in form. It would not be 

 unnatural to expect that these early animals and plants would 

 prove to be very primitive in their structure and low in the 

 scale of evolution, but such is not the case. Of the eight 

 or nine primary divisions of the animal kingdom, all but the 

 highest, the vertebrates, have Cambrian representatives. It 

 is probably not too much to say that more than one half of 

 the development of the animal kingdom was accomplished 

 before the Cambrian. We thus get a hint of the long ages 

 which preceded the time of which the geologic record gives us 

 an intelligible story. In spite of the great development of 

 life before the Cambrian, enormous progress was made in the 

 later periods, and, as compared with the animals which suc- 

 ceeded them, the Cambrian types show many primitive 

 characteristics. 



Plants existed. Concerning the plants of the Cambrian 

 time, little is known ; but since plants provide the ultimate 

 food supply of most animals, it is evident that they must 

 have been then in existence. We may perhaps attribute 

 the lack of fossils to the fact that the Cambrian rocks thus 

 far studied are of marine origin, and most marine plants are 

 too soft and succulent to be readily preserved as fossils. Only 

 when in younger strata we come to the deposits made in 

 marshes and rivers by the plants which possess woody tissues, 

 do we find vegetable remains well preserved. 



The more prominent animals of the Cambrian. Two 

 groups of animals, the brach'iopods and the trilobites, have 

 left fossil remains in such abundance that they are regarded 

 as the most important of all that numerous assemblage of 

 species which is called the Cambrian fauna. The early 

 brachiopods had pairs of small oval or rounded shells which 



