PALAEONTOLOGY 77 



species had originated quite independently of the 

 others. There are a few exceptions to this rule, and 

 these cases of ' ' discontinuous distribution ' ' are 

 accounted for by the extinction of the species in 

 certain parts of the area and not in others. Also 

 there is always a close connection between the 

 present fauna of a district and its fauna in the 

 Pleistocene and Pliocene periods, intimating that 

 one is descended from the other. 



The proof by Gradation. By this is meant the 

 exhibition of a series of fossils, arranged according 

 to their age, shewing the gradual change of one form 

 into another. Of course the acknowledged imper- 

 fection of the record forbids us from expecting to 

 find many of these cases, or of very long lines of 

 gradation ; still there are some. This is especially 

 the case in the Brachiopods, some of the Ammonites, 

 and the horse ; and in a more broken line with the 

 Crocodiles. But I will take the Camel as an 

 example. The genus Camelus appears first in the 

 lower Pliocene and is preceded by the closely allied 

 Procamelus in the upper Miocene. Pcebrotherium, 

 of the lower Miocene, connects this form with the 

 primitive Artiodactyles of the Oligocene ; and these 

 are connected through the Condylarthra with the 

 early carnivora, and these with the primitive insecti- 

 vora and polyprotodont metatheria. Then comes a 

 break between the metatheria and the prototheria ; 

 but the latter are closely related to the theriodont 

 reptiles. Indeed, Professor H. G. Seeley at first 

 thought that Theriodesmus was a mammal ; and it 

 was only the discovery of Pareiasaurus that proved 



