50 PRINCIPLES OF PALZONTOLOGY. 
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 paleontological 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 paleontological record, due 
