THE EVOLUTION OF THE MAMMALIA. 41 
preserved, or if preserved, to be found in a much less complete 
state than those which are larger and have bones which are bet- 
ter able to resist crushing. Gregarious animals, living in herds, 
are, from their greater individual numbers, much more likely to be 
fossilized than animals of solitary habit, and thus hoofed animals 
are more numerous as fossils than flesh-eaters or insectivores. 
Arboreal animals escape many of the catastrophes which over- 
whelm the species that live on the ground, and flying forms are 
especially fitted to escape them. Bats are, therefore, very rare 
as fossils, and their history is almost unknown. For these reasons, 
the history of the hoofed animals, and especially of those of con- 
siderable stature, is the best known of all. 
One of the most striking and significant results of the study 
of the later Mesozoic and earliest Tertiary mammalian faunas, is 
that the higher or placental mammals are seen to be converging 
back to a common ancestral group of clawed and carnivorous or 
omnivorous animals, now entirely extinct, to which the name of 
Creodonta was given by Cope. The creodonts are assuredly the an- 
cestors of the modern flesh-eaters, and, very probably, of the great 
series of hoofed animals also, as well as of other orders. From this 
central, ancestral group the other orders proceed, diverging more 
and more with the progress of time, each larger branch dividing 
and subdividing into smaller and smaller branches, until the 
modern condition is attained. Many a stately branch that at 
one time seemed full of vigor and promise of long life has withered 
and dropped from the trunk, while insignificant twigs have 
expanded into great and ramified branches. The vicissitudes of 
the history are many and surprising, and, in many instances, 
quite inexplicable from the standpoint of present knowledge. 
The stem-group for the hoofed animals and that which serves 
to connect the latter with the creodonts, is the Condylarthra, 
an extinct group which also was discovered and named by 
the late Professor Cope. The Condylarthra probably began 
their career in the late Mesozoic ; at all events, they are 
numerous in the Puerco and Torrejon, and die out in 
the Wasatch. Though the connection has not yet been 
