Concerning their external characteristics we know much; and our classifications, if not satisfactory to all, are at least eminently useful.  But when one turns to the morphological sciences of anatomy, histology, embryology, and pathology, one discovers great gaps, where knowledge might reasonably be expected.  Even gross anatomy has much to gain from the careful, systematic examination of these organisms.  With still greater force this statement applies to the studies of finer structural relations.  Little is known concerning the embryological development and life history of certain of the primates, and almost nothing concerning their pathological anatomy.

Clearly less satisfactory than our knowledge of structure is the status of information concerning those functional processes which are the special concern of physiology and pathology.  Certain important experimental studies have been made on the nervous system, but rarely indeed have physiologists dealt systematically with the functions of other systems of organs.  There are almost no satisfactory physiological descriptions of the monkeys, anthropoid apes, or lower primates.

SUB-DIVISIONS OF THE ORDER PRIMATES

Order Sub-orders Families

,- a.  PROSIMII (Lemurs and Aye-Ayes)
|
|                       ,-  i.  Hapalidae (Marmosets)
|                       |  ii.  Cebidae (Howling Monkeys,
PRIMATES -+                       |       Tee Tees, Squirrel Monkeys,
|                       |       Spider Monkeys, and Capuchin
|                       |       Monkeys)
`- b.  ANTHROPOIDEA ... -+ iii.  Cercopithecidae (Baboons
|       and Macaques)
|  iv.  Simiidae (Gibbons, Orangs,
|       Chimpanzees, and Gorillas)
`-  v.  Hominidae (Man)

When we turn to the science of genetics we meet a similar condition, for the literature reveals only scattered bits of information concerning heredity in the primates.  No important experimental studies along genetic lines have been made with them, and such general observations from nature as are on record are of extremely uncertain value.  Were one to insist that we know nothing certainly concerning the relation of heredity in other primates than man, the statement could not well be disputed.

Occasionally in recent years students of human diseases have employed monkeys or apes for experimental tests, but aside from the isolated results thus obtained, extremely little is known concerning the diseases peculiar to the various types of infra-human primates or the significant relations of their diseases to those of man.

Next in order of extent to our morphological knowledge of these organisms is that of their behavior, mental life, and social relations.  But certainly no one who is conversant with the behavioristic, psychological and sociological literature could do otherwise than emphasize its incompleteness and inadequacy.  For our knowledge of behavior has come mostly from naturalistic observation, scarcely at all from experimentation; our knowledge of social relations is obviously meager and of uncertain value; and finally, our knowledge of mind is barely more than a collection of carelessly drawn inferences.