126 CHARLES DARWIN 



of this long-projected treatise. The second part, as he 

 told Mr. Fiske, was to have treated of the variation of 

 animals and plants through natural selection ; while the 

 third part would have dealt at length with the pheno- 

 mena of morphology, of classification, and of distribu- 

 tion in space and time. But these latter portions of 

 the work were never written. To say the truth, they 

 were never needed. So universal was the recognition 

 among the younger men of Darwin's discovery, that 

 before ten years were over innumerable workers were 

 pushing out the consequences of natural selection into 

 every field of biology and palaeontology. It seemed no 

 longer so necessary as it had once seemed to write the 

 larger and more elaborate treatise he had originally 

 contemplated. 



The volume on the variation of animals and plants 

 contained also Darwin's one solitary contribution to the 

 pure speculative philosophy of life his 'Provisional 

 Hypothesis of Pangenesis,' by which he strove to 

 account on philosophical principles for the general facts 

 of physical and mental heredity. Not to mince matters, 

 it was his one conspicuous failure, and is now pretty 

 universally admitted as such. Let not the love of the 

 biographer deceive us ; Darwin was here attempting a 

 task ultra vires. As already observed, his mind, vast 

 as it was, leaned rather to the concrete than to the 

 abstract side : he lacked the distinctively metaphysical 

 and speculative twist. Strange to say, too, his abor- 

 tive theory appeared some years later than Herbert 

 Spencer's magnificent all-sided conception of ' Physio- 

 logical Units,' put forth expressly to meet the self-same 

 difficulty. But while Darwin's hypothesis is rudely 



