NA TURA L HIS TORY OF CREA TION. 1 6 1 



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This diagram shows only the main ramifications ; but the 

 reader must suppose minor ones, representing the sub- 

 ordinate difterences of orders, tribes, famiUes, genera, etc., 

 if he wishes to extend his views to the 

 wdiole varieties of being in the animal 

 kingdom. Limiting ourselves at present 

 to the outline afforded by this diagram, 

 it is apparent that the only thing re- D 

 quired for an advance from one type 

 to another in the generative process is 

 that, for example, the fish embryo C" 

 should not diverc^e at A, but c^o on to A 

 C before it diverges, in which case the 

 progeny will be, not a fish, but a rep- 

 tile. To protract the straiyhtforioard 

 ]jart of the yestation over a small space — and from species 

 to species the space would be small indeed — is all that 

 is necessary. 



This might be done by the force of certain extern;d 

 conditions operating upon the parturient system. The 

 nature of the conditions we can only conjecture, for 

 their operation, which in the geological eras was so 

 powerful, has in its main strength been long interrupted, 

 and is now perhaps only allowed to work in some of the 

 lowest departments of the organic world, or under extra- 

 ordinary casualties in some of the higher, and to these 

 points the attention of science has as yet been little 

 directed. But though this knowledge were never to be 

 clearly attained, it need not much affect the present 

 argument, provided it be satisfactorily shown that there 

 must be some such influence within the range of natural 

 things. 



To this conclusion it must be greatly conducive that 

 the law of organic development is still daily seen at woi'k 



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