Il8 THE LIVING WORLD. 



now are, to be sure, different from those found in 

 the Silurian (2), but the genera are identical, and the 

 differences between the species are not great. In 

 such cases the same species may exist from one age 

 to another. These are. however, exceptional in- 

 stances. Generally the particular species character- 

 istic of each age were replaced in the subsequent 

 age by a new set, and this same fact is also true of 

 the genera. Families were longer lived, and many 

 families living in the Silurian (2) age inhabit our 

 seas to-day. 



It is not common, then, for any species to succeed 

 in outliving the long periods between the geological 

 ages. Nevertheless, the different species of animals 

 have had quite different lengths of life. Some are 

 confined to a single stratum of a single age, while 

 others extend through two or three geological sys- 

 tems (some Brachiopoda). It is found also to be 

 a general rule that the lowest species are of the 

 longest duration. For instance, certain species of 

 Foraminifera (Saccaminina Carteri) appeared in the 

 Silurian, and have continued to exist until to-day 

 .with little or no modification. So, too, with the 

 Lamellibranchiata and Crustacea, the lowest forms 

 appeared early and continued to live a long time, 

 while the higher ones were more rapidly modified. 

 Again, as a somewhat better illustration, we find that 

 in the Tertiary (9), the reptiles had already assumed 

 the forms which have continued to exist up to the 

 present time. Not so, however, with the higher 

 vertebrates, which have undergone great changes 

 since that time. The same is true everywhere. Low 



