PRINCIPLES AND METHODS OF PALAEONTOLOGY. 383 



G. There arc yet one or two collateral points wliicb require discussion. 

 Supposing tliat tlie present bed of the ocean were upheaved and be- 

 came exposed to view, so that we could examine the organic contents of 

 all the strata of mud and sand which have accumulated and hardened 

 into stone for the last four or five thousand years, ought we to expect to 

 find, at any one spot, a complete and unbroken series of the remains of 

 all the creatures that have ever lived there 1 Assuredly not. In the 

 first place, it has already been explained that there are many animals 

 entirely devoid of parts sufficiently hard to be i)reservable, and of them 

 every trace would have disappeared. It is important to remark that a 

 naturalist who should have become acquainted with the present animal 

 creation only in this way, would be ignorant of the existence of many 

 genera and families, of some orders, and even of one or two entire 

 classes; but no sub-kingdom would be without abundant representa- 

 tives, and, therefore, he would be perfectly acquainted with all the great 

 types of organization at present existing. There Avould necessarily b& 

 defects in his knowledge, but these defects would by no means interfere 

 with his obtaining a very clear and just, though not complete, idea of 

 the present state of things. 



But there are other and more formidable sources of imperfection in 

 our palaeontological knowledge. Not only does the very nature of some 

 animals present an insuperable bar to the preservation of a complete 

 record of organic life in the rocks contemporaneously formed, but it is, 

 to saj' the least, excessively improbable that a complete series of even 

 those organic bodies which are preservable should be found at any one 

 spot. For modern research teaches that the level of the land is con- 

 stantly changing ; slowly but surely, some conntries are rising, while 

 others are becoming depressed ; and there is good evidence that, in some 

 parts of the world, several alternative movements of elevation and de- 

 l^ression have taken place within a comparatively modern period. IS'ow, 

 whenever the bottom of the sea becomes dry land, or the dry land sinks 

 to the bottom of the sea, there must obviously be an interruption in the 

 series of living inhabitants, aquatic forms replacing terrestrial, or vice 

 versa. Thus, should the sea bottom be raised into dry land, and then 

 depressed again so as to be covered with fresh de])osits, the whole mass, 

 when subsequently elevated and exposed to view, will exhibit a break 

 in the series of marine organic remains, corresi>onding in magnitude 

 and importance with the interval during which the sea bed remained in 

 the condition of dry land. It is probable that there is not a single spot 

 on the earth's surface which has not been thus subjected to many altera- 

 tions of elevation and depression, and, hence, we may safely infer that 

 no single series of superimposed strata can contain a complete series of 

 even those forms of past life which have flourished in that one region. 



But, if this be true of those marine animals whose chances of preser- 

 vation are greatest, whose hard parts contain so little animal matter as 

 to be not worth attack on the part of predacious organisms, which are 

 sufficiently dense to resist the destructive agencies to which they must 

 almost necessarily l)e exposed before they are protected by sediment, 

 and whose locomotive powers are insufficient to enable them to escape 

 by migration the imminent fate threatened, by changes of level, how 

 much more fortuitous nuist be the preservation of those remains which, 

 like the bones of the marine Verfebrata, contain much animal matter, 

 and are comparatively soft, or which belong to entirely terrestrial crea- 

 tures. And, in fact, it is among the rarest of occurrences to find the 

 bones of a dead wild quadruped, or bird ; or to dredge up from the sea 

 bottom a relic of a fish or of a porpoise, abundant as these animals are 

 in our seas. 



