206 CONTINUITY. 



( Paleontologists have endeavoured to separate the Lias 

 into a number of subdivisions, by the Ammonites, groups of 

 species of those shells being characteristic of different zones. 

 The evidence on this point rests on the assumption of specific 

 differences being indicated by permanent modifications of the 

 structure of the shell. But it is quite possible that these may 

 mean nothing more than would be due to some change in the 

 conditions of existence. Except between the Marlstone and 

 the Upper Lias there is really no palaeontological break, in 

 the proper sense of the words; alterations of form and size 

 consequent on the occurrence of circumstances more or less 

 favourable, migration of species, and other well-known causes, 

 sufficiently account for many of those modifications of the 

 form of the shell that have been taken as specific marks. This 

 view is strengthened by the fact that other shells and other 

 organisms generally show no proof of a break of any import- 

 ance except at the point already alluded to.' 



But, irrespectively of another deficiency in the geological 

 record, which will be noticed presently, the physical breaks in 

 the stratification make it next to impossible to fairly trace the 

 order of succession of organisms by the evidence afforded by 

 their fossil remains. Thus there are nine great breaks in the 

 Palaeozoic series, four in the Secondary, and one in the 

 Tertiary, besides those between Palaeozoic and Secondary and 

 Secondary and Tertiary respectively. Thus, in England there 

 are sixteen important breaks in the succession of .strata, to- 

 gether with a number of less important interruptions. But 

 although these breaks exist, we find pervading the works of 

 many geologists a belief, resulting from the evidence presented 

 to their minds, sometimes avowed, sometimes unconsciously 

 implied, that the succession of species bears some definite 

 relation to the succession of strata. Thus, Professor Ramsay 

 says, that ' in cases of superposition of fossiliferous strata, in 

 proportion as the species are more or less continuous, that is 

 to say, as the break in the succession of life is partial or com- 

 plete, so was the time that elapsed between the close of the 

 lower and the commencement of the upper strata a shorter 

 or a longer interval. The break in life may be indicated not 

 only by a difference in species, but yet more importantly by 



