only be preserved during those periods in which long-continued slow
subsidence is going on. This in turn, led to the still wider and more
suggestive conclusion that the geological record as a whole is, and never
can be more than, a series of more or less isolated fragments. The
recognition of this important fact constitutes the keystone to any theory
of evolution which seeks to find a basis in the actual study of the types
of life that have formerly inhabited our globe.

In his third chapter, Darwin gives a number of interesting facts, collected
during his visits to the plains and valleys of Chili, which bear on the
question of the origin of saliferous deposits--the accumulation of salt,
gypsum, and nitrate of soda. This is a problem that has excited much
discussion among geologists, and which, in spite of many valuable
observations, still remains to a great extent very obscure. Among the
important considerations insisted upon by Darwin is that relating to the
absence of marine shells in beds associated with such deposits. He justly
argues that if the strata were formed in shallow waters, and then exposed
by upheaval to subaerial action, all shells and other calcareous organisms
would be removed by solution.

Following Lyell's method, Darwin proceeds from the study of deposits now
being accumulated on the earth's surface, to those which have been formed
during the more recent periods of the geological history.

His account of the great Pampean formation, with its wonderful mammalian
remains--Mastodon, Toxodon, Scelidotherium, Macrauchenia, Megatherium,
Megalonyx, Mylodon, and Glyptodon--this full of interest. His discovery of
the remains of a true Equus afforded a remarkable confirmation of the fact-
-already made out in North America--that species of horse had existed and
become extinct in the New World, before their introduction by the Spaniards