STRUCTURAL MODIFICATIONS 145 



of the gaps which divide them into species thenceforth incapable 

 of intermingling, had nothing to do with it. On the contrary, 

 it would have rendered this differentiation impossible. 



The causes which determined the formation of the four great 

 divisions resulting from these modifications, moreover, were in- 

 dependent of one another. Since all four sprang from theWorms, 

 their formation must have been simultaneous, and this enables 

 us to understand that, however far back we may go, these are 

 always to be found in association : except perhaps for a 

 slight delay in the appearance of the Vertebrates, which is 

 explained by the fact that they are the result, not of purely 

 chemical phenomena, but of the degree of perfection attained 

 by the nervous system. 



These embranchements have persisted because the causes to 

 which they were due at first remained constant. Gradually, how- 

 ever, they became disengaged from these causes, and new causes, 

 essentially intrinsic and constituting what we call heredity, 

 were substituted for those that were primitively operative ; 

 and these hereditary determining factors are still sufficient to 

 perpetuate the fundamental forms they created. Later 

 modifying agencies have only been able to effect changes in 

 detail. 



It was then, as we have seen, in the water, and certainly not 

 below the depth of four hundred metres reached by the really 

 useful solar rays, and particularly along the shores of seas where 

 all that is necessary for them in the way of food abounds, that 

 living forms became diversified. We must now inquire 

 how these forms descended to the depths of the sea and how 

 they came to enrich the solid earth. 



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