56 INTRODUCTION. 



there was no such sudden destruction of life at the close of each 

 great geological epoch, and no such creation of fresh forms at the 

 commencement of the next period. On the contrary, it is certain 

 that there is a geological "continuity," such as we see in other 

 departments of nature, and that the lines which we draw between 

 the great formations merely mark periods of time in which no rocks 

 were laid down, or the rocks deposited in which are at present 

 unknown to us. 



What are we to believe occurred at the close of any great geo- 

 logical period — say, the Cretaceous period ? If we reject the view 

 that the close of the period was marked by a sudden and universal 

 extinction and destruction of the characteristic Cretaceous forms of 

 life, there is only one other view which we can take. Confining our 

 attention solely to those seas of the period of which alone we know 

 enough for safe reasoning, we know that the close of the Cretaceous 

 period in Europe was accompanied, or rather caused, by an upheaval 

 of the Cretaceous area, and an obliteration of the Cretaceous sea. 

 This upheaval was, of course, effected with extreme slowness, or, at 

 any rate, not suddenly, and it must have completely changed the 

 life-conditions or " environment " of the animals which swarmed in 

 the Cretaceous seas. Some of these would doubtless be unable to 

 accommodate themselves to their altered surroundings, and would 

 simply die out. Others, we may presume, would migrate to some 

 more favourable area, and some of these might accomplish their 

 migration without undergoing any change. Most of the forms which 

 migrated, in the process of migration, and by reason of coming into 

 contact with strange neighbours and untried conditions, would, 

 however, probably undergo more or less modification. Ultimately, 

 therefore, many characteristic Cretaceous forms might be transferred 

 to some sea far distant from their original home. Not only so, but 

 some of the transferred species might have suffered so much modifi- 

 cation that they would no longer be regarded as specifically identical 

 with the original Cretaceous forms, but would be looked upon simply 

 as allied or " representative " species, though really the lineal de- 

 scendants of the animals of the Chalk. 



It is perfectly clear that the process of rock-deposition which was 

 going on in Europe towards the close of the Cretaceous period was 

 not, and could not be, abolished by the elevation of the European 

 area, and the obliteration of the Cretaceous sea, but was simply 

 transferred to some other area. In this particular case, we do not 

 happen to know where the new area of deposition may have been. 

 It is quite certain, however, that in whatever area the Cretaceous 

 animals took refuge, there rocks must have been deposited in course 

 of time, as they are in all seas, though it does not in the least follow 

 that the rocks of this new era should have the smallest likeness in 



