130 ORGANIC EVOLUTION CONSIDERED 



been discovered. From these remains it is evident 

 that marsupials of many kinds must „have abounded 

 in the Jurassic. 



It is also evident that the progenitors of these Tri- 

 assic marsupials must have begun as mammals, per- 

 haps as monotremes, at some period in the Paleozoic. 

 As to how the transitions could have taken place from 

 reptile to monotreme, from monotreme to marsupial, 

 and from marsupial to placental mammal, we will not 

 at present inquire. 



The oldest known mammals are not the lowest in 

 structure, and, from their long history antecedent to 

 their oldest remains thus far known, we may expect 

 to find their remains in Paleozoic rocks, if they have 

 been evolved from lower forms. 



From the abundance of the remains of marsupial 

 mammals in the Jurassic and the numerous kinds of 

 placental mammals in the Eocene, we would expect 

 that, in the Cretaceous, which intervenes between the 

 Jurassic and the Eocene, would be found the remains 

 of many marsupial and placental mammals, together 

 with transitional forms between them. It is, how- 

 ever, a remarkable fact that nearly the whole of the 

 enormous deposits of the Cretaceous, amounting to 

 38,000 feet in thickness,* are, so far as had been dis- 

 covered up to 1894, destitute of the remains of mam- 

 mals. Not a single placental mammal has been found 

 in the Cretaceous. Up to 1894 only one specimen of 

 a mammal had been found in the foreign Cretaceous, 

 and that was in the Wealden, the lowest part of the 

 Cretaceous period. The other remains of mammals 

 in the Cretaceous have been found in the upper part, 

 while none have been discovered in the deposits of 

 great thickness intervening between the remains in 

 the lower and upper parts. 



* Manual of Geology, Dana, pp. 818 and 828. 



