Throughout the long ages of the Tertiary period monkeys must have been
very abundant and very varied, yet it is but rarely that their fossil
remains are found. This, however, is not difficult to explain. The
deposits in which mammalian remains most abound are those formed in
lakes or in caverns. In the former the bodies of large numbers of
terrestrial animals were annually deposited, owing to their having been
caught by floods in the tributary streams, swallowed up in marginal bogs
or quicksands, or drowned by the giving way of ice. Caverns were the
haunts of hyenas, tigers, bears, and other beasts of prey, which dragged
into them the bodies of their victims, and left many of their bones to
become embedded in stalagmite or in the muddy deposit left by floods,
while herbivorous animals were often carried into them by these floods,
or by falling down the swallow-holes which often open into caverns from
above. But, owing to their arboreal habits, monkeys were to a great
extent freed from all these dangers. Whether devoured by beasts or birds
of prey, or dying a natural death, their bones would usually be left on
dry land, where they would slowly decay under atmospheric influences.
Only under very exceptional circumstances would they become embedded
in aqueous deposits; and instead of being surprised at their rarity
we should rather wonder that so many have been discovered in a fossil
state.

Monkeys, as a whole, form a very isolated group, having no near
relations to any other mammalia. This is undoubtedly an indication of
great antiquity. The peculiar type which has since reached so high a
development must have branched off the great mammalian stock at a very
remote epoch, certainly far back in the Secondary period, since in the
Eocene we find lemurs and lemurine monkeys already specialized. At this
remoter period they were probably not separable from the insectivora,