76 General Part. 



VIII. The Geological Distribution of 

 Animals. 



G-eology, tlie history of the evolution of the earthy shows that 

 animal life has existed during immeasurable periods of time; 

 and further that the fauna through these ages has by no means 

 been always the same as at present, but has been subject to a series 

 of extraordinarily great changes. The animal remains of 

 different periods, enclosed in the earth's crust, are the source of 

 this knowledge. 



The presence of such, traces in the earth's crust is intelligible from the 

 folio-wing considerations. In all natural waters, but especially in the sea, there 

 is a constant deposition of finer or coarser particles, which may be either in 

 soKition or in suspension in the water : the waves, assisted by atmospheric 

 action, tear off portions of the coast, and the debris is laid down again, the 

 coarser sand or gravel at once, in the shallower places ; the very fine, which is 

 set in motion by the slightest ripple, only at greater depths, where the waves 

 do not penetrate. Rivers carry down to the sea great quantities of alluvium, into 

 which sink the dead shells of numberless minute marine organisms : in this way 

 the sand, gravel, or mud of the ocean bed, forms compressed deposits, which, in 

 course of time, often harden into solid masses of shale, limestone or sandstone. 

 The deposits are stratified: the end of one stratum and the beginning of 

 another are important as indicating either an inten-uption in the sedimentation 

 or the beginning of a different formation. The same thing occui-s on a small 

 scale in great inland seas. 



Such a formation of strata has taken place from the earliest times, and 

 in so far as animals have lived in water, or have chanced to fall in after death, 

 their remains ai-e imbedded m the deposits. These stoi-es of animal remains 

 would, however, avail little, were they inaccessible; but the great changes 

 which have occurred in the earth's surface in the course of ages and which 

 are always taking place, have put the beds, even of oceans and of inland seas, 

 within reach. Parts of the surface, which in earlier times were covered by 

 the sea, are now, in consequence of elevations, dry land, and therefore may 

 be investigated, and with great facility, for at many places and in many 

 ways natural sections of the strata have been produced, so that remains 

 biuied in the bed of the sea at different epochs have been brought to light. 

 The same thing has often occun-ed in fresh- water beds. 



The animal remains which have been found in the earth's strata 

 are, as a rule, grouped under the general term of petrifactions 

 or fossils. The former name is, however, not absolutely correct, 

 for "petrified" remains are only exceptionally found. As a rule, only 

 the hardest of the calcified parts have been preserved ; traces of the 

 soft parts occur quite rarely, and then usually as impressions in the 

 stone, which must in these cases be of a very fine grain : impressions 

 of Hydrozoa, for example, are sometimes found in the Bavarian 

 lithographic slate, originally a calcareous silt. The hard parts 

 occurring in the strata are in many cases relatively unaltered ; the 



