EXTINCT SPECIES. 



THE study of geology teaches us that our planet has under- 

 gone many successive physical revolutions; the crust of it 

 being made up of layer upon layer, after the manner of the 

 successive peels of an onion. Each of these successive deposi- 

 tions constitutes the tomb of animal forms, that have lived 

 and passed away. 



Now it is a fresh-water or a marine shell that the explor- 

 atory geologist discloses ; now the skeleton, or parts of a 

 skeleton bones, from the evidence of which a comparative 

 anatomist can reproduce, by model or picture, the exact ori- 

 ginal forms. Occasionally Science has to build up her pre- 

 sentment of animals that were from the scanty evidence of 

 their mere footfalls. As the poacher is guided to the timid 

 hare, crouching in her seat, by the vestiges of footprints on 

 the snow, so the geologist can, in many cases, arrive at toler- 

 ably certain conclusions relative to the size and aspect of an 

 extinct animal by the evidence of footsteps on now solid rock. 

 If it be demanded how it happens that now solid rocks can 

 bear the traces of such soft impressions, the reply is simple. 

 There evidently was a time when these rocks, now so hard 

 and solid, were mere agglomerations of plastic matter com- 

 parable for consistence to ordinary clay. It needs not even 

 the weight of a footfall to impress material of temper so soft 

 as this. The plashes of rain are distinctly visible upon many 

 rocks now hard, and which have only acquired their consist- 

 ence with the lapse of countless ages. 



