44 THE GEOLOGICAL EVIDENCES 



expression of necessity for a change ; hence, the 

 rotation of forms. Among the amphibians, rep- 

 tiles, and birds, likewise, we observe that the older 

 forms are very different from those now living, 

 but the difference becomes less and less marked 

 as we approach the present day. The same 

 holds equally true with the mammals, whose ear- 

 liest representatives are again forms of a very low 

 grade of organization. These are the marsupials 

 of the Triassic and Jurassic periods, forms more 

 or less closely allied to some of the lowly types 

 inhabiting the Australian continent. 



The chart before you indicates the rise and fall of 

 this highest order of animals. It will be seen that 

 they date their first appearance from the Triassic 

 period, where, however, there are but three or four 

 genera, and a barely larger number of individuals, 

 represented. One of these, and the first species 

 described, is on the table before you, known as 

 Dromatherium. A further development takes place 

 in the Jurassic period, when a broad hiatus fol- 

 lows. No mammalian remains have thus far 



