THE PRE-ADAMITE EARTH. 



526. The number of orders of which examples are found depo- 

 sited in the strata are as follows : 



Palaeozoic formation . 31 

 Triassic . . . . 21 

 Jurassic 41 



Cretaceous . . . 41 

 Tertiary . . .71 

 Existing period . . . 77 



These figures show that tfie orders of animals are in constantly 

 increasing progression. The exception presented by the Triassic 

 group may probably be explained by the want of sufficient 

 observation of its organic remains. 



527. This constant increase does not prevail with the genera. 

 In each period each order consisted of a certain definite number 

 of genera. Now, on comparing the number of genera of which 

 each of these orders consisted, from period to period, it will be 

 found that while some continually increased, others having in- 

 creased up to a certain date, attained their maximum, and then 

 decreased, and that the decrease has continued to the present 

 time. 



528. Of the seventy-seven orders, it has been found by com- 

 parison of the organic remains of the successive periods, that sixty- 

 three have had a continually increasing number of genera from 

 their first appearance to the present epoch, but that the numbers 

 of the genera of the other fourteen respectively after increasing to 

 a certain period, then decreased, and that this decrease continued. 



529. Six of these fourteen orders acquired their greatest gene- 

 ric development in the Palaeozoic period. These six consisted 

 of two orders of Fishes, one of Crustacea, two of Mollusca, and 

 one of Hadiata. 



530. The two orders of Fishes are those denominated by 

 naturalists the placoids and the ganoids. Of the former the shark 

 and the ray, and of the latter the sturgeon, are living examples. 



144 



