350 PROGRESS OF SCIENCE IN THE CENTURY. 



that only certain rocks are suitable for preserving 

 their enclosures, and that many rocks have been un- 

 made and remade in the course of ages. As we walk 

 along the shore and study the jetsam, we see how 

 quickly many of the sea's memoranda are lost 



On the other hand, we must not exaggerate the 

 imperfection; indeed, the biologist has often much 

 reason to be gratefully surprised at the reverse. 

 Many fossil jelly-fishes most unlikely subjects of 

 preservation are known, and have been carefully 

 studied, e.g., by Haeckel and by Walcott Some- 

 times a whole series can be followed, and the transi- 

 tions from species to species studied, as in the case 

 of fresh-water beds containing shells of Paludina and 

 Planorbis. On a larger scale, Hyatt's tracking of 

 the evolutionary paths of the Ammonites is a monu- 

 mental piece of work. In some cases, even in Grap- 

 tolites, a little palax>ntological embryology, or study 

 of young forms at least, is possible. Half a dozen 

 unborn young may be seen inside an ichthyosaurus 

 in the museum at Stuttgart and the remains of 

 belemnites may be counted in the stomach. Some- 

 times in a fossil fish there is not a bone a-missing or 

 out of place, though very much the reverse is the 

 rule. 



It is difficult to have much satisfaction in the 

 fragmentary remains (skull-cap, femur, and two 

 teeth) of Pithecanthropus erecttu found by Dubois 

 (1894) in what were regarded as Upper Pliocene de- 

 posits in Java. The remains may be those of a 

 transitional form between man and his unknown 

 simian ancestors, but the evidence is by no means 

 sufficient But, in other cases, the preservation is 

 so perfect that certain conclusions may be arrived 

 at The skeleton of Phenacodus, carefully studied 



