SELECTION 159 



of character, with extinction of intermediate 

 forms, explains the difficulties of classification 

 of making a genealogical tree which will 

 express the facts of the case and represent 

 diagrammatically " the great tree of life, 

 which fills with its dead and broken branches 

 the crust of the earth and covers the sur- 

 face with its ever-branching and beautiful 

 ramifications." 



DARWIN'S SUMMARY. The preceding sum- 

 mary of the classical statement of the doctrine 

 of natural selection should be supplemented 

 by reference not only to the original work, to 

 the corroborative labours of its author, to the 

 able independent treatise ("Natural Selec- 

 tion ") of Wallace, and to the synthetic treat- 

 ments of the whole subject of evolution given 

 by Haeckel in his "Generelle Morphologic," 

 and by Spencer in his " Principles of Biology," 

 but to the enormous mass of exposition, 

 argument and illustration accumulated by 

 subsequent writers, commencing with Hooker 

 and Asa Gray, Huxley and Haeckel, but soon 

 becoming too numerous for mention. We 

 indicate, however, a few convenient recent 

 summaries in the "Bibliography" at the close 

 of this volume. 



But while we must avoid the error of sup- 

 posing that the last word on natural selection 



