laid in successive strata at the bottom of the sea. Each of these
sedimentary strata was at first a soft layer of mud; but in the course
of thousands of years it condensed into a solid, hard mass of stone
(sandstone, limestone, marl, etc.), and at the same time permanently
preserved the solid and imperishable bodies that had chanced to fall
into the soft mud. Among these bodies, which were either fossilised or
left characteristic impressions of their forms in the soft slime, we
have especially the more solid parts of the animals and plants that
lived and died during the deposit of the slimy strata.

Hence each of the sedimentary strata has its characteristic fossils,
the remains of the animals and plants that lived during that
particular period of the earth's history. When we make a comparative
study of these strata, we can survey the whole series of such periods.
All geologists are now agreed that we can demonstrate a definite
historical succession in the strata, and that the lowest of them were
deposited in very remote, and the uppermost in comparatively recent,
times. However, there is no part of the earth where we find the series
of strata in its entirety, or even approximately complete. The
succession of strata and of corresponding historical periods generally
given in geology is an ideal construction, formed by piecing together
the various partial discoveries of the succession of strata that have
been made at different points of the earth's surface (cf. Chapter
2.18).

We must act in this way in constructing the phylogeny of man. We must
try to piece together a fairly complete picture of the series of our
ancestors from the various phylogenetic fragments that we find in the
different groups of the animal kingdom. We shall see that we are
really in a position to form an approximate picture of the evolution