CHAPTER VIIL 



HOW IS ORGANIC EVOLUTION CAUSED? 



§ 143. Already it has been necessary to speak of the 

 causes of organic evolution in general terms; and now we 

 are prepared for considering them specifically. The task 

 before us is to affiliate the leading facts of organic evolu- 

 tion, on those same first principles conformed to by evolution 

 at large. 



Before attempting this, however, it will be instructive to 

 glance at the causes of organic evolution which have been 

 from time to time alleged. 



§ 144. The theory that plants and animals of all kinds 

 were gradually evolved, seems to have been at first accom- 

 panied only by the vaguest conception of cause — or rather, 

 by no conception of cause properly so called, but only by the 

 blank form of a conception. One of the earliest who in 

 modern times (1735) contended that organisms are indefi- 

 nitely modifiable, and that through their modifications they 

 have become adapted to various modes of existence, was 

 De Maillet. But though De Maillet supposed all living 

 beings to have arisen by a natural, continuous process, he 

 does not appear to have had any definite idea of that which 

 determines this process. In 1794, in his Zoonomia, 



Dr. Erasmus Darwin gave reasons (sundry of them valid 

 ones) for believing that organized beings of every kind, have 

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