3o8 



ESSENTIALS OF BIOLOGY 



the evolution of any groups of animals or plants : it only suggests 

 the methods of representing in a diagram the successions, in time, 

 of the kinds of animals found as fossils and their structural relations 

 to each other. 



What we are really justified in doing is to make such a diagram- 

 matic representation of fossil records as that which follows : 



CaxTLoxoijc 

 era. 



Mesozoic 



Paleozoic 

 ProteTozoic 



OLTUL 



Archeozoic 



Fig. 40. — The Graphical Representation of Paleontological Facts. 



We find fossil remains of animals. A, that are all so structurally 

 similar to each other as to justify us in including them all in the 

 phylum, or groups of allied species, called A. These fossils are 

 found in all the rocks of the stratigraphical series and we conclude 

 that there has been an unbroken series of generations persistent 

 since the upper Proterozoic times. Similarly with B and C. 

 From the end of the Paleozoic periods we find fossils all of 

 which so resemble each other that we group them together in 

 the class D. But the structure of these animals D is rather like 

 that of the phylum C : indeed we regard them as included in the 

 diagnosis of C, but they are a special sub-group, or class, of C 

 and so we make the line representing them converge towards C. 

 So also with the line E, which represents a group of species, the 

 structure of w^hich is such that D includes it, but it is also such 

 as to be regarded as a special sub-group, or order of D. So we 

 make it converge towards D. These convergences are based on 

 morphological similarities, as made out by study of fossil structure, 



