378 0. C. Marsh— Vertebrate Life in America. 
changes took place in the limb bones. One result was a great 
increase in speed, as the power was applied so as to act 
only in the plane of motion. The best effect of this speciali- 
zation is seen to-day in the Horse and Antelope, each repre- 
senting a distinct group of Ungulates, with tive-toed ancestors. 
If the history of American Mammals as I have briefly 
sketched it, seems as a whole incomplete, and unsatisfactory, 
we must remember that the genealogical tree of this class has its 
trunk and larger limbs concealed beneath the débris of Meso- 
zoic time, while its roots doubtless strike so deeply into the 
Paleozoic that for the present they are lost. A decade or two 
hence, we shall probably know something of the mammalian 
fauna of the Cretaceous, and the earlier lineage of our existing 
mammals can then be traced with more certainty. 
The results I have presented to you are mainly derived 
from personal observation ; and since a large part of the higher 
vertebrate remains found in this country have passed through 
my hands, I am willing to assume full responsibility for my 
presentation of the subject. 
r our present knowledge of the extinct Mammals, Birds 
and Reptiles of North America, science is especially indebted 
to Leidy, whose careful, conscientious work has laid a secure 
foundation for our vertebrate paleontology. The energy of 
Cope has brought to notice many strange forms, and greatl 
brates have been described by Lund, Owen, Burmeister, Ger- 
vais, Huxley, Flower, Desmarest, Aymard, Pictet, and Nodot. 
Darwin and Wallace have likewise contributed valuable infor- 
mation on this subject, as they have on nearly all forms of life. 
In this long history of ancient life I have said nothing of 
what Life itself really is. And for the best of reasons, because 
I know nothing. Here at present our ignorance is dense, and 
yet we need not despair. Light, Heat, Electricity, and Magnet- 
ism, Chemical Affinity and Motion, are now considered different 
forms of the same force; and the opinion is rapidly gaining 
ground that Life, or vital force, is only another phase of the 
ower. Possibly the great mystery of Life may thus be 
solved, but whether it be or not, a true faith in Science knows 
no limit to its search for Truth. 
