180 GEOLOGICAL DISTRIBUTION. 



be said to be broadly marked out. Thus, in the Australian Pleis- 

 tocene marsupials, Diprotodon, Nototherium, Thylacoleo, and their 

 allies, we have the forerunners of the various marsupial forms that 

 now characterise the continental fauna ; in the giant birds Palapte- 

 ryx, Dinornis, Mionornis, &c., from New Zealand, and ^Epyornis 

 from Madagascar, the forerunners of the wingless apteryx and the 

 struthious birds from the same or neighbouring regions; and in 

 the giant South American edentates, Glyptodon, Megatherium, My- 

 lodon,* and their allies, the representative, if not the ancestral, 

 forms of the existing sloth, armadillo, and ant-eater. It is re- 

 markable that these last forms also occur in the Post-Pliocene de- 

 posits of the United States ; but there can be but little doubt that 

 their presence there was the result of a temporary migration from 

 the south, since their remains are only exceptionally found in any 

 of the preceding Tertiary formations. 



We have noted in the Eocene period the presence of a certain 

 generalised group of mammals, from which, by gradual modifica- 

 tions of structure, the more specialised groups of subsequent periods 

 have sprung. The demonstration of this successive evolution of 

 forms is not, however, restricted solely to groups of animals, but it 

 can be indicated with no less positiveness in the case of certain in- 

 dividual members of a group. The most notable instance of evolu- 

 tionary modification in a given line is afforded by the horse. Thus, 

 from the modern horse we can trace downward in the geological 

 scale a gradual series of modifications in the structure of the teeth 

 and limbs which, at the further end of the line, characterise an 

 animal so far removed in general structure from the existing form 

 that, were not the intermediate forms known, or were it to be con- 

 sidered by itself, it would be recognised not only as the type of a 

 distinct family, but of a distinct sub-order. From the solidungu- 

 late type represented in the existing form we reach, by sensible 

 gradations, an animal of the polydactyl type, or one having several 

 toes to each foot. A phylogenetic line, but little less complete, can 

 also be traced in connection with the families Camelida?, Tapiridee, 

 and Felidse, and others, doubtless, will be discerned with the fur- 

 ther progress of paleontological investigation, 



* The deposits containing these remains have been very generally consid- 

 ered to be Post-Pliocene ; Arneghino and Cope, however, probably correctly 

 refer them either in whole, or in part, to the Pliocene. 



