58 INTRODUCTION. 



present covered by the ocean. Lastly, we must not forget that 

 there may have been times in which great changes in life were 

 actively progressing in areas in which there might be little or no 

 contemporaneous deposition of rock, so that the extreme terms of 

 a series might be preserved to us whilst all the intermediate links 

 might have escaped record. 



From these and similar causes, it is certain that we shall rarely 

 be able to point to a complete series of deposits linking one great 

 geological period, such as the Cretaceous, to another, such as the 

 Eocene. Still, we may well have a strong conviction that such 

 deposits must exist, or must have existed, as memorials of, at any 

 rate, part of the time which elapsed between the close of the one 

 formation and the commencement of the next. Upon any theory 

 of " evolution," at any rate, it is certain that there can be no total 

 break in the great series of the stratified deposits, but that there 

 must have been a complete continuity of life, and a more or less 

 complete continuity of deposition, from the Cambrian period to the 

 present day. There was, and could have been no such continuity 

 in any one given area ; but the chain could never have been snapped 

 at one point and taken up at a wholly different one. It remains 

 certain, however, that we can never dispense with the division of 

 the stratified series into definite rock-groups and life-periods. We 

 can never hope to discover all the lost links of the geological chain, 

 and the great geological systems will always be separated from one 

 another by more or less evident physical or palseontological breaks, 

 or by both combined. The utmost we can at present do is to 

 arrive at the conviction that the lines of demarcation between the 

 great formations only mark gaps in our knowledge, and that there 

 can in nature be no hiatus in the long series of fossiliferous deposits. 



Life - Zones. 



While each geological rock-system is characterised by a general 

 assemblage of distinctive types of animals and plants, the minor sub- 

 divisions of each system are likewise distinguished by the prevalence 

 of particular forms of life. There are, no doubt, cases in which an 

 extensive series of successive strata may appear to be characterised 

 throughout by essentially the same organic types, there being appar- 

 ently no restriction of special fossils to special horizons in the series. 

 In so far as such cases have any real existence, they may be ex- 

 plained as instances in which a great series of sediments has been ac- 

 cumulated with such rapidity that there has been no time for marked 

 biological changes, resulting in the dying out of old species and the 

 introduction of new forms. In many cases, however, the apparent 

 diffusion of the same kinds of fossils from the base to the summit of 



