CONTEMP-OKANEITY OF STRATA. 47 



Nowadays, most geologists hold that 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 commence- 

 ment of the next period. On the contrary, they hold 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 

 geological 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, however, of the forms which 

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

 into contact with strange neighbours and untried conditions, 

 would probably undergo more or less modification. Ulti- 

 mately, 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 modification 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 

 descendants of the animals of the Chalk. 



