Ch. XXVII.] EARLIEST KNOWN VERTEBRATA. 59^ 



vertebrata, would follow partly from our having as palaeontologists to 

 do chiefly with strata of marine origin, and partly because bones of 

 fish, however partial and capricious their distribution on the bed of 

 the sea, are nevertheless more easily met with than those of reptiles 

 or mammalia. In like manner the extreme rarity of birds in Recent 

 and Pliocene strata, even in those of freshwater origin, might lead us 

 to anticipate that their remains would be obtained with the greatest 

 difficulty in the older rocks, as the Table proves to be the case — even 

 in tertiary strata, wherein we can more readily find deposits formed 

 in lakes and estuaries. 



The only incongruity between the geological results and those 

 which our dredging experiences might have led us to anticipate 

 a priori, consists in the frequency of fossil reptiles, and the com- 

 parative scarcity of mammalia. It would appear that during all the 

 secondary periods, not even excepting the newest part of the creta- 

 ceous, there was a greater development of reptile life than is now wit- 

 nessed in any part of the globe. The preponderance of this class 

 over the mammalia may have depended in part on climatal condi- 

 tions, but it seems also clearly to imply the limited development, if 

 not the total absence, before the Tertiary period, of the placental 

 mammalia, whether terrestrial or aquatic, which, when they became 

 dominant, acquired power to check and keep down the class of verte- 

 brata nearly allied to them in structure, and coming most directly in 

 competition with them in the struggle for life. For notwithstanding 

 the impossibility of assigning even conjectural limits to the chrono- 

 logical extension of each class of vertebrata as we trace them farther 

 and farther back into the past, it cannot be denied that our failure to 

 detect signs of them in older strata, in proportion to the rank of their 

 organization, favors the doctrine of development, or at least of the 

 successive appearance on the earth of beings more and more highly 

 organized, culminating at last in the advent of Man himself. 



