206 The Study of Animal Life part in 



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 fossilised 

 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 {Pahtdina), 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 progress- 

 ive 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 an extinction 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 descendants 

 of Trilobites and Eurypterids, or of many of the ancient 

 reptiles. There is no doubt that a race may die out. 

 Many different kinds of heavily armoured Ganoid fishes 

 abounded in the ages when the Old Red Sandstone was 

 formed, but only seven different kinds are now alive. 

 The lamp-shells and the sea-lilies, once very numerous, are 

 now greatly restricted. Once there were giants among 

 Amphibians, now almost all are pigmies. 



