RESULTS OF PAL^ONTOLOGICAL RESEARCH 29 



The forms which most nearly approach the chief 

 reptilian order of the present day turtles, crocodiles, 

 and serpents appeared later. 



The Birds are known to us through the two Archae- 

 opteryx from the Solnhof Slate (upper Jura). Remains 

 of birds, as generally of all land animals, can naturally 

 be but seldom preserved. The two Archseopteryx tell us 

 practically nothing at all of the history of the Birds. 



The Mammalia appear, as a class regarded generally, 

 for the first time in the upper Trias l and in forms which 

 nearly approach the lowest orders of the class of Cloaca 

 and Marsupials. 3 



The higher orders appear later, but then certainly 

 and simultaneously and partly in the most differentiated 

 forms such as the whale (Cetacese) bats, and Probo- 

 scidae (Tertiary in the Eocene period). 3 



Conclusions from (2) : The higher classes of the 

 Vertebrates appear after the lower (Birds ?) ; within the 

 classes also the higher orders appear later than the lower. 



Many groups (it might perhaps be said of all, were 

 the evidence more perfect) show always at their first 

 appearance already a division at least into some higher 



1 The Mammalia were therefore older than the Birds, the first remains 

 of which come from the later Jura, if the Archseopteryx may not be 

 regarded as shattered examples which have reached us altogether by 

 chance. Possibly the birds are much older. 



2 Marsupials and Cloaca are primitive forms because, since with them 

 the development of the embryos is entirely with the Cloaca, or mainly, as 

 with the Marsupials extra-uterine. With the higher orders a placenta is 

 formed, and the development is entirely inter-uterine. 



3 Further particulars concerning the classes of Vertebrates are to be 

 found in G. Steinmann's Die Geologischen Grundlagen der Abstammungslehre, 

 p. 203. 



