86 ANIMALS OF THE PAST 
change has taken place in them since that 
time; they seem to have become so exactly 
adapted to the conditions of existence that no 
further modification has taken place. This 
may be expressed in another way, by saying 
that while the Mammals of the Eocene have 
no near relatives among those now living, 
entire large groups having passed completely 
out of existence, the few birds that we know 
might, so far as their appearance and affinities 
go, have been killed yesterday. 
‘Were we to judge of the former abundance 
of birds by the number we find in a fossil 
state, we should conclude that in the early 
days of the world they were remarkably scarce, 
for bird bones are among the rarest of fossils. 
But from the high degree of development evi- 
denced by the few examples that have come 
to light, and the fact that these represent 
various and quite distinct species,* we are led 
* But three birds, besides a stray feather or two, are so far 
known from the Eocene of North America. One of these is a 
fonl not very unlike some of the small curassows of South 
America ; another is a little bird, supposed to be related to the 
sparrows, while the third is a large bird of uncertain relation- 
ships. 
