INTRODUCTION. 13 
or anatomy of birds; Osteology , or the science of bones; 
and Odontography, or the natural history of teeth. 
Systematic Zoology is the classification or grouping of ani¬ 
mals according to their structural and developmental rela¬ 
tions. The systematic knowledge of the several classes, 
as Insects, Reptiles, and Birds, has given rise to subordinate 
sciences, like Entomology , Herpetology , or Ornithology'* 
Distributive Zoology is the knowledge of the successive ap¬ 
pearance of animals in the order of time (Paleontology in 
part), and of the geographical and physical distribution of 
animals, living or extinct, over the surface of the earth. 
Theoretical Zoology includes those provisional modes of 
grouping facts, and interpreting them, which still stand 
waiting at the gate of science. They may be true, but we 
cannot say that they are true. The evidence is incomplete. 
Such are the theories which attempt to explain the origin of 
life and the origin of species. 
Suppose we wish to understand all about the Horse. Our 
first object is to study its structure. The whole body is en¬ 
closed within a hide, a skin covered with hair ; and if this 
hide be taken off, we find a great mass of flesh or muscle, 
the substance which, by its power of contraction, enables 
the animal to move. On removing this, we have a series of 
bones, bound together with ligaments, and forming the skel¬ 
eton. Pursuing our researches, we find within this frame¬ 
work two main cavities : one, beginning in the skull and 
running through the spine, containing the brain and spinal 
marrow ; the other, commencing with the mouth, contains 
the gullet, stomach, intestines, and the rest of the apparatus 
for digestion, and also the heart and lungs. Examinations 
of this character would give us the Anatomy of the Horse, 
or, more precisely, Hippotomy. The study of the bones 
alone would be its Osteology ; the knowledge of the nerves 
would belong to Neurotomy. If we examined, under the 
microscope, the minute structure of the hair, skin, flesh, 
blood, and bone, we would learn its Histology. The consid¬ 
eration of the manifold changes undergone in developing 
from the egg to the full-grown animal, would be the Embry- 
* Numbers like this refer to the Notes at the end of the volume. 
