402 RETROGRADE MOVEMENT OF REl'TILIA. chap. xx. 



and islands are tenanted by about seventeen hundred species 

 of mammalia, of which only forty-six are marsupial, namely, 

 the opossums of North and South America. But the great 

 difference of age of the strata in each of these three localities 

 seems to indicate the predominance throughout a vast lapse 

 of time (from the era of the Upper Trias to that of the 

 Purbeck beds) of a low grade of quadrupeds; and this per- 

 sistency of similar generic and ordinal tjqjes in Europe while 

 the species were changing, and while the fish, reptiles, and 

 mollusea were undergoing vast modifications, raises a strong 

 presumption that there was also a vast extension in space of 

 the same marsupial forms during that portion of the secondary 

 ejioch which has been termed "the age of reptiles." 



As to the class Eeptilia, some of the orders which pre- 

 vailed when the secondary rocks were formed are confessedly 

 much higher in their organization than any of the same 

 class now living. If the less perfect ophidians, or snakes, 

 which now abound on the earth had taken the lead in those 

 ancient days among the land reptiles, and the Deinosaurians 

 had been contemjiorary with Man, there can be no doubt 

 that the progressionist would have seized upon this fact with 

 unfeigned satisfaction as confirmatory of his views. Now 

 that the order of succession is precisely reversed, and that 

 the age of the Iguanodon was long anterior to that of the 

 Eocene palseophis and living boa, while the crocodile is in 

 our own times the highest representative of its class, a retro- 

 grade movement in this important division of the vertebrata 

 must be admitted. It may perhaps be accounted for by the 

 power acquired by the placental mammalia, when they 

 became dominant, a j)ower before which the class of verte- 

 brata next below them, as coming most directly in com- 

 petition with them, may, more than an}' other, have given 

 way. 



For no less than thirty-four j-ears it had been a received 



