THE EVOLUTIONARY CAREER 



307 



in characters from A that we are justified in classifying them as 

 a different family of organisms from that one to which A belonged. 

 And so on with orders and classes. We may therefore simplify 

 the last diagram as in 4, Fig. 38, and make the convention that 

 a straight, unbroken line represents a family, or order, or class 

 according to the minuteness of our classification. 



If these methods were perfectly justified by the evidence (fossil 

 records) that we have we might now construct phylogenies, or 

 schemes of descent, showing the actual ways in which the various 

 groups of animals have evolved from each other and from a 

 common, original stock. Thus : 



Fig. 39. — An Imaginary Phylogeny. 



w^here the basal line A represents an original stock. From A 

 a group of species, all resembling each other in certain common 

 characters, has evolved : we call this group of species the class 

 of animals B. This class consists of many species, each specific 

 line really representing a great succession of generations of 

 individuals. C is another such divergence from A. D and E 

 represent two groups of species into which the common stock 

 A has broken up and so on. The whole diagram might be said 

 to be the representation of the phylogeny of the existing (in 

 Cainozoic times) groups of animals F-K. It is, of course, a 

 pure fiction and it may be said at once, that the records of paleonto- 

 logy do not enable us to construct such a scheme as applying to 



