5O PRINCIPLES OF PALEONTOLOGY. 



that the lapse of time, indicated by the unconformability, has 

 been sufficiently great to allow of the dying out or modifica- 

 tion of many of the older forms of life, and the introduction of 

 new ones by immigration. 



Apart, however, altogether, from these great physical breaks 

 and their corresponding breaks in life, there are other reasons 

 why we can never become more than partially acquainted with 

 the former denizens of the globe. Foremost amongst these is 

 the fact that an enormous number of animals possess no hard 

 parts of the nature of a skeleton, and are therefore incapable, 

 under any ordinary circumstances, of leaving behind them any 

 traces of their existence. It is true that there are cases in 

 which animals in themselves completely soft-bodied are never- 

 theless able to leave marks by which their former presence can 

 be detected. Thus every geologist is familiar with the wind- 

 ing and twisting " trails " formed on the surface of the strata 

 by sea -worms; and the impressions left by the stranded 

 carcases of Jelly-fishes on the fine-grained lithographic slates 

 of Solenhofen supply us with an example of how a creature 

 which is little more than "organised sea -water" may still 

 make an abiding mark upon the sands of time. As a general 

 rule, however, animals which have no skeletons are incapable 

 of being preserved as fossils, and hence there must always 

 have been a vast number of different kinds of marine animals 

 of which we have absolutely no record whatever. Again, 

 almost all the fossiliferous rocks have been laid down in water; 

 and it is a necessary result of this that the great majority of 

 fossils are the remains of aquatic animals. The remains of 

 air-breathing animals, whether of the inhabitants of the land 

 or of the air itself, are comparatively rare as fossils, and the 

 record of the past existence of these is much more imperfect 

 than is the case with animals living in water. Moreover, the 

 fossiliferous deposits are not only almost exclusively aqueous 

 formations, but the great majority are marine, and only a com- 

 paratively small number have been formed by lakes and rivers. 

 It follows from the foregoing that the palaeontological record 

 is fullest and most complete so far as sea-animals are concerned, 

 though even here we find enormous gaps, owing to the absence 

 of hard structures in many great groups ; of animals inhabiting 

 fresh waters our knowledge is rendered still further incomplete 

 by the small proportion that fluviatile and lacustrine deposits 

 bear to marine ; whilst we have only a fragmentary acquaint- 

 ance with the air-breathing animals which inhabited the earth 

 during past ages. 



Lastly, the imperfection of the palaeontological record, due 



