SELECTION 159 



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 doc- 

 trine of natural selection should be supple- 

 mented by reference not only to the original 

 work, to the corroborative labours of its 

 author, to the able independent treatise 

 ("Natural Selection") of Wallace, and to the 

 synthetic treatments of the whole subject of 

 evolution given by Haeckel in his "Generelle 

 Morphologic," and by Spencer in his "Prin- 

 ciples of Biology, ' ' but to the enormous mass 

 of exposition, argument and illustration 

 accumulated by subsequent writers, com- 

 mencing with Hooker and Asa Gray, Huxley 

 and Haeckel, but soon becoming too numer- 

 ous for mention. We indicate, however, 

 a few convenient recent summaries in the 

 "Bibliography 5 at the close of this volume. 



But while we must avoid the error of sup- 

 posing that the last word on natural selection 

 was said by Darwin, or that there is not still 

 abundant opportunity both for reflection 

 and research in regard to it, we must be 

 clear as to the essential simplicity of the 

 general theory. Darwin himself summed it 

 up in a couple of sentences: "As many more 

 individuals of each species are born than can 



