f the
earth consists of wild and uncivilised regions in which no exploration
of the rocks has been yet made, so that whether we shall find the
fossilised remains of any particular group of animals which lived during
a limited period of the earth's history, and in a limited area, depends
upon at least a fivefold combination of chances. Now, if we take each of
these chances separately as only ten to one against us (and some are
certainly more than this), then the actual chance against our finding
the fossil remains, say of any one order of mammalia, or of land plants,
at any particular geological horizon, will be about a hundred thousand
to one.

It may be said, if the chances are so great, how is it that we find such
immense numbers of fossil species exceeding in number, in some groups,
all those that are now living? But this is exactly what we should
expect, because the number of species of organisms that have ever lived
upon the earth, since the earliest geological times, will probably be
many hundred times greater than those now existing of which we have any
knowledge; and hence the enormous gaps and chasms in the geological
record of extinct forms is not to be wondered at. Yet, notwithstanding
these chasms in our knowledge, if evolution is true, there ought to have
been, on the whole, progression in all the chief types of life. The
higher and more specialised forms should have come into existence later
than the lower and more generalised forms; and however fragmentary the
portions we possess of the whole tree of life upon the earth, they ought
to show us broadly that such a progressive evolution has taken place. We
have seen that in some special groups, already referred to, such a
progression is clearly visible, and we will now cast a hasty glance over
the entire series of fossil forms, in order to see if a similar
progression is manifested by them as a whole.

_The Progressive Development of Plants._

Ever since fossil plants have been collected and studied, the broad fact
has been apparent that the early plants--those of the Coal
formation--were mainly cryptogamous, while in the Tertiary deposits the
higher flowering plants prevailed. In the intermediate secondary epoch
the gymnosperms--cycads and coniferae--formed a prominent part of the
vegetation, and as these have usually been held to be a kind of
transition form between the flowerless and flowering plants, the
geological succession has always, broadly spe