ORGANIC EVOLUTION 



237 



understand that the tracing out of natural phyla, even with 

 abundant material , is a matter of great difficulty, and that 

 when forms are insufficient and relationships not clear it 

 admits of great diversity of opinion, and makes errors of 

 interpretation easy. 



The divergence of development stated in the systematic 

 table on page 221 may be more graphically presented to the 

 mind if the groups contained therein be arranged in such a 

 diagram as is shown in figure 146. Such graphic represen- 

 tations of the possible course of evolution have been much 

 used in the past, in spite of their purely hypothetical 

 character; and although less commonly employed now, 

 still they are an excellent aid to the mind in grasping the 

 idea of genetic relationships. 



FIG. 146. A genealogic tree ; a graphic mode of illustrating possible relation- 

 ship between organisms. 



