CHAP. XVIII.] 
BIRDS. 
371 
not come into existence, and preserved only in those areas 
which were long free from the incursions of such dangerous 
enemies. The discovery of Struthious remains in Europe in the 
Lower Eocene only, supports this view ; for at this time carnivora 
were few and of generalized type, and had probably not acquired 
sufficient speed and activity to enable them to exterminate 
powerful and quick-running terrestrial birds. It is, however, at 
a much more remote epoch that we may expect to find the 
remains of the earlier forms of this group ; while these Eocene 
birds may perhaps represent that ancestral wide-spread type 
which, when isolated in remoter continents and islands, became 
modified into the American and African ostriches, the Emeus 
and Cassowaries of Australia, the Dinornis and JE^yornis of 
New Zealand, 
B B 2 
