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 nevertheless able to leave marks by which their former presence can be detected: Thus every geologist is familiar with the winding 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 comparatively small number have been formed by lakes and rivers. It follows from the foregoing that the palæontological 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 acquaintance with the air-breathing animals which inhabited the earth during past ages.