BREAKS IN THE GEOLOGICAL RECORD. 51 



find two formations to be unconformable, we shall always find 

 at the same time that there is a great difference in their fossils, 

 and that many of the fossils of the older formation do not sur- 

 vive into the newer, whilst many of those in the newer are not 

 known to occur in the older. The cause of this is, obviously, 

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

 been sufficiently great to allow of the dying out or modification 

 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 " organized 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 1s 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 comparatively 

 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 struc- 



