A VIEW IN PERSPECTIVE. 117 



this long series of ages there has been a constant 

 change of species. Few species of animals have 

 succeeded in remaining in existence very long ; or, it 

 is probably better to say, few species of animals 

 have succeeded in breeding true for a very long 

 time. We are, of course, speaking geologically, and 

 thus are reviewing time by ages, rather than by 

 years. Sooner or later the descendants of any one 

 species died or became so changed from their earlier 

 form as to constitute a new species. Thus through- 

 out the past new species have been continually 

 appearing. It is indeed very seldom that any spe- 

 cies continues to exist from one age to another, and 

 thus, as a rule, the species of each age were distinct. 

 As already noticed, there were long periods between 

 the geological ages, during which no record of life 

 has been preserved. These blank periods were un- 

 doubtedly periods of considerable disturbance in 

 nature, and they lasted many thousands of years. 

 The only record we have of the events that occurred 

 during these times is in the modification which they 

 produced upon the living world. The disturbances 

 were usually sufficient to change animal forms so 

 much as to produce an entirely new set of species. 

 It is thus usually possible to determine closely the 

 age of any fossil by its specific characters. 



(Nevertheless, we find that no universal rule can 

 be given ; for while most species change with the 

 advent of new ages, some of the species of animal, 

 were much more persistent. Some forms of life 

 remained with little change through all the geologi- 

 cal periods. This is true of Lingula, some foramini- 

 fers, and some mollusks. \ The species that are found 



