EVOLUTION-PAST AND PRESENT 



629 



Fig. 25-6. This is a reconstruction of a Devonian coral reef that existed in the region of Rochester, New York. Note 

 the great variety of form and structure of these ancient coelenterates. In the foreground on the right is a huge 

 trilobite, a distant relative of the Crustacea. In the middle foreground are two cephalopod mollusks, one in a 

 straight cone-like shell and the other in a segmented coiled shell. All of these species are extinct today. 



neously and simultaneously in a large va- 

 riety of forms. Assuming that they did, all 

 groups would very shortly acquire the char- 

 acter or perish, in accordance with the law 

 of selection. This might conceivably ac- 

 count for the sudden appearance of the 

 ancestors of all modern phyla in tliis very 

 early period of geologic history. 



Most, if not all, of the major phyla have 

 left fossil remains in the Cambrian, which 

 lasted 60 million to 90 million years, cer- 

 tainly a sufficiently long period of time to 

 allow for the evolution of such a large va- 

 riety of forms. Each of the principal phyla 

 represents a distinct level of anatomical 

 organization which had its beginnings then. 

 Apparently by happy coincidence, specific 

 anatomical and physiological characteristics 

 appeared that possessed great evolutionary 

 potentialities. These became evident in the 

 diversification of species within each phy- 

 lum that followed. The course that each has 

 taken from the Cambrian up to the present 



is portrayed in Fig. 25-5 in graphic form. It 

 will be seen that each of the principal phyla 

 today started in the Cambrian and through 

 the subsequent periods has had its "ups and 

 downs." Moreover, in the past geologic pe- 

 riods the relative proportions of the various 

 groups differ from those of today (Figs. 

 25-6, 25-7). The most striking fact that 

 emerges from this observation is that all 

 of the original phyla have living representa- 

 tives today and that all of them are more 

 numerous than in Cambrian times. Not only 

 are there more individuals, but there is also 

 a much greater variety of species. This is 

 one of the remarkable outcomes of organic 

 evolution — more and more animals with 

 greater and greater diversity of fonn and 

 structure. To say it another way, there is a 

 never ending trend toward producing more 

 and more protoplasm organized in more 

 and more different ways. 



With the phyla well established in the 

 Cambrian, let us follow the history of one, 



