CHAPTEE XXIII. 



THE CAUSES OF DEVELOPMENT, 



How has 

 the traus- 

 mutation 

 of species 

 beeu 

 caused ? 



How has 

 organic 

 structure 

 been pro- 

 duced ? 



We must 

 hegin by 

 trying 



TN the foregoing chapters I have stated a mass of cumu- 

 -*- lative evidence which, when taken together, amounts, 

 as I believe, to a conclusive proof that species have not 

 been independently created ; but that all the species of the 

 same fundamental type certainly, and all organic species 

 whatever iwobahly, have been deocended from the same 

 original stock. I regard it as certain, for instance, that all 

 the Vertebrata are descended from one ancestor, and all 

 the Annulosa from another ; and so among plants with 

 Endogens and Exogens. And I regard it as highly pro- 

 bable that those ancestors, in their turn, were descended 

 from a still older stock, which was the original germ of all 

 organisms whatever. I believe that in the chapter on 

 Embryology, especially, I have pointed out the course that 

 has been followed by the successive changes which have 

 developed some of the lower forms into higher forms of 

 the same fundamental type. But I have as yet scarcely 

 approached the question, by what agency the development 

 of one species out of another by descent has been brought 

 about. And if it is true that all organisms are descended 

 from a few original vitalized though unorganized germs, 

 the further question arises, how the matter of those 

 original germs was caused to assume an organic structure ; 

 or, in other words, by what causes, acting on vitalized 

 matter, organic structure is produced? 



It is a carduial principle in scientific reasoning to begin 

 by trying whether known causes are adequate to account for 

 the phenomena under investigation; and it is only when 



