The Pkinciples of Palaeontology. 161 



A curious instance, recently brought to light, is that of the 

 pineal eye of Reptiles There is found among certain Lacer- 

 tilians, on the top of the head and on the median line, an organ 

 which in cases of the greatest differentiation has the structure of 

 an eye, with retina, crystalline humor and optic nerve which passes 

 through a perforation of the parietal bone. But this organ is 

 concealed under an opaque scale, and in no case can be used for 

 sight. It is, moreover, o^enerally very small. 



But a number of Reptiles of the Primary and Secondary epochs, 

 especially those of the lower groups, present a parietal opening 

 situated exactly like that of the Lizards, with a much larger aper- 

 ture. It appears then almost certain that at that epoch the 

 pineal eye must have fallilled the function of an eye, and its 

 presence is quite inexplicable in actual types where its situation 

 is such that it can not serve for seeing, unless we allow the 

 admission that these existing forms have descended from ancient 

 types where this organ served a useful purpose. It is a curious fact, 

 moreover, that the animal in which the pineal eye is the least 

 reduced, the genus Hatteria, belongs to the most ancient group 

 known in the whole class of Reptiles (Rhynchocephala). 



To sum up this subject, the existence of rudimentary organs is 

 one of the most conclusive arguments in favor of the theory of 

 Evolution. 



•Parallelism and Convergence. — The attentive study of the 

 variations of organs among forms living or fossil, has brought to 

 light another important phenomenon which in a marked degree 

 restricts the importance of the principle of Cuvier regarding the 

 correlation of forms. It has been observed that in some groups, 

 whether .allied or very diverse, the series of modifications was 

 produced in the same method and along parallel lines. Further, 

 in certain cases, if we examine through successive strata forms 

 originally dissimilar, we find that they evolve in such a manner as 

 to diminish their differential characters, so that the derived forms 

 of each series resemble each other much more than do primitive 

 forms. These are the phenomena of Convergence. 



In regard to forms very closely allied, it is natural that we 

 should find similar conditions producing similar modifications. 

 Natural selection, or the direct influence of the medium, suffices 

 to explain this. Thus, in very extensive basins, the Paludinas, 

 though smooth and with inflated volutions, have, at various 

 epochs and different points, evolved into carinated and tubercu- 

 lous forms. 



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