348 THE STUDY OF ANIMAL LIFE CHAP. 



are sedimentary rocks formed under water, often they 

 have been unmade and remade, burnt and denuded. The 

 chances against preservation are many. 



Soft animals rarely admit of preservation, those living 

 on land and in the air are much less likely to be preserved 

 than those living in water, the corpses of animals are 

 often devoured or dissolved. Again the chances against 

 preservation are many. 



3. Palaeontological Series. Imperfect as the geological 

 record is, several marvellously complete series of related 

 animals have been disentombed. Thus, a series of fossil- 

 ised freshwater snails (Planorbis) has been carefully 

 worked out ; its extremes are very different, but the 

 distinctions between any two of the intermediate forms 

 are hardly perceptible. The same is true in regard to 

 another set of freshwater snails (Paludina), and on a much 

 larger scale among the extinct cuttlefishes (Ammonites, 

 etc.) whose shells have been thoroughly preserved. The 

 modern crocodiles are linked by many intermediate forms 

 to their extinct ancestors, and the modern horse to its 

 pigmy progenitors. In cases like these, the evidences of 

 continuously progressive evolution are conclusive. 



4. Extinction of Types. A few animals, such as some 

 of the lamp shells or Brachiopods, have persisted from 

 almost the oldest rock-recorded ages till now. In most 

 cases, however, the character of the family or order or 

 class has gradually changed, and though the ancient forms 

 are no longer represented, their descendants are with us. 

 There is a disappearance of individuals and a slow change 

 of species. 



On the other hand there are not a few fossil animals 

 which have become wholly extinct, whose type is not 

 represented in the modern fauna. Thus there are no 

 animals alive that can be regarded as the lineal descen- 

 dants of Trilobites and Eurypterids, of relatively simple 

 forms, such as Graptolites, which may have been Zoo- 

 phytes, or of highly specialised types like the Flying 

 Dragons or Pterodactyls. There is no doubt that a race 

 may die out. Of the many different kinds of heavily 

 armoured Ganoid fishes which abounded in the ages when 



