156 ELEMENTS OF BIOLOGY. 



ing a given area of the sea-bottom, and we suppose the con- 

 ditions of that area to be changed for the worse, either by 

 an elevation of the sea-bottom or from any other cause, a 

 migratio?i of the fauna will be set on foot. The locomotive 

 animals will shift their quarters in search of some other area 

 in which the conditions are more favourable to their exist- 

 ence. As sedentary animals have almost universally loco- 

 motive young, we may from this point of view regard all the 

 animals of such an area as capable of migrating. A general 

 migration of the fauna of the area will commence, and in 

 this way some of the species of the area will be transferred 

 to another area. By a repetition of this process the same 

 species may ultimately come to inhabit an area removed by 

 a hemisphere from its original habitat ; and in this way the 

 same species may present itself in beds at the most distant 

 parts of the earth's surface. 



It is quite clear, however, from the above, that the iden- 

 tity of fossils in widely distant strata, is, upon the whole, a 

 proof that the beds in question are jaot strictly contempo- 

 raneous. A migration is a work of time, and one of the two 

 sets of beds must obviously and necessarily be younger 

 than the other by the period consumed in the migration. 

 Still the interval between two such sets of beds would not 

 be long, geologically speaking, and both groups of strata 

 would belong approximately to the same geological horizon. 

 If, therefore, we still apply the name of " contemporaneous'' 

 to beds which contain the same fossils but are widely 

 separated from one another in point of distance, we must 

 do so on the clear understanding that the term must be 

 taken in a wider and looser sense than that in which it is 

 ordinarily employed. 



Geological Continuity. — The entire series of Stratified 

 or Fossiliferous rocks, as before remarked, admits of a natu- 

 ral division into a certain number of definite "rock-groups" 

 or " formations," each of which is characterised by a peculiar 

 and distinctive assemblage of fossils, constituting the "life" 



