34 THE SCIENTIFIC EVIDENCES OF 



sight. But now, suppose that this had not been 

 the case, and that the two structures, besides 

 presenting the necessary superficial resemblance, 

 had also presented an anatomical resemblance ; 

 with what tremendous force might it have then 

 been urged, — "Your hypothesis of hereditary de- 

 scent with progressive modification being here 

 excluded, by the fact that the animals compared 

 belong to two widely different branches of the 

 tree of life, how are we to explain the identity of 

 type manifested by these two complicated organs 

 of vision ? The only hypothesis open to us is 

 intelligent adherence to an ideal type." But as 

 this cannot now be urged in any one case through- 

 out the whole organic world, we may, on the other 

 hand, present it as a most significant fact, that, 

 while within the limits of the same large branch 

 of the tree of life we constantly find the same 

 typical structures modified so as to perform very 

 different functions, we never find any vestige of 



