VIII. Geological Distribution of Animals. 77 



shells of MolluscSj Echinoderms, etc.; are often but little changed, 

 so also are the bones of Vertebrata, except that the organic sub- 

 stances disappear, and only calcareous matters remain, so that fossil 

 bones are lighter than fresh ones. In other cases, the bones or 

 shells have been infiltrated with other matters, such as the silica 

 dissolved in the water. For instance, Bchinoid shells of the 

 Cretaceous period are found, filled with silicates (flint) ; or silicated 

 bones, as hard as stone, penetrated throughout with silica. To 

 such cases, the term " petrified " may be correctly applied. Often 

 the hard parts themselves are not found, but merely their 

 impressions in the stone in which they were imbedded. Some- 

 times the shells, originally filled with silica, have later been dissolved 

 away, so that only the cast remains, a mass of flint with an impression 

 of the interior of the shell on its surface. 



It is evident that the conception of the fauna of early times to 

 be formed from the study of fossils (Paleeontology), must be 

 very incomplete. Only an extremely small proportion of the animals 

 of a given period are preserved in deposits; by far the greater number 

 disappear without a trace. Of these there are, in the first place, all 

 those which possess no skeleton, so that they vanish entirely with 

 few exceptions. Of the rest, the terrestrial forms are preserved only 

 in favourable cases, for, all those which are left after death, on dry 

 land, soon disappear entirely. Aquatic animals, especially marine 

 forms, with hard structures in or round them, stand, on the other 

 hand, a better chance of being preserved ; but of these, by far 

 the greater number vanish, even when they are not devoured by 

 other animals, for by no means every part of the ocean bed is fit 

 for their permanent preservation. As for the parts which, in earlier 

 times, were really imbedded in a suitable place, and so were preserved, 

 many have been lost again, for the strata have been disturbed by 

 volcanic action, for example, which destroys all traces of life.- 

 Pinally, it must be noticed that only an extremely small proportion 

 of the materials contained in the strata now, is accessible to the 

 investigator : most of it is only too well preserved. All this must be 

 taken into consideration in estimating the importance of the con- 

 ception of the fauna of byegone ages, supplied by the examination 

 of fossils. 



Now it is clear, from this inquiry, that animal life has gradually 

 undergone great changes from the oldest times until now ; the 

 different periods (see p. 79), into which the history of the earth 

 may be divided, were characterized by different floras and faunas 

 whose remains are preserved in the strata, where they have been 

 ever since. The more remote the time, the more different is the 

 animal (and plant) life from that now existing. The animals found 

 in the older formations may, for the most part, be classified without 

 difficulty to-day, in the phyla and classes arranged for existing forms. 



