of absolute degradation
or degeneration. Serpents, for example, have been developed from some
lizard-like type which has lost its limbs; and though this loss has
enabled them to occupy fresh places in nature and to increase and
flourish to a marvellous extent, yet it must be considered to be a
retrogression rather than an advance in organisation. The same remark
will apply to the whale tribe among mammals; to the blind amphibia and
insects of the great caverns; and among plants to the numerous cases in
which flowers, once specially adapted to be fertilised by insects, have
lost their gay corollas and their special adaptations, and have become
degraded into wind-fertilised forms. Such are our plantains, our meadow
burnet, and even, as some botanists maintain, our rushes, sedges, and
grasses. The causes which have led to this degeneration will be
discussed in a future chapter; but the facts are undisputed, and they
show us that although variation and the struggle for existence may lead,
on the whole, to a continued advance of organisation; yet they also lead
in many cases to a retrogression, when such retrogression may aid in the
preservation of any form under new conditions. They also lead to the
persistence, with slight modifications, of numerous lowly organised
forms which are suited to places which higher forms could not fully
occupy, or to conditions under which they could not exist. Such are the
ocean depths, the soil of the earth, the mud of rivers, deep caverns,
subterranean waters, etc.; and it is in such places as these, as well as
in some oceanic islands which competing higher forms have not been able
to reach, that we find many curious relics of an earlier world, which,
in the free air and sunlight and in the great continents, have long
since been driven out or exterminated by higher types.

_Summary of the first Five Chapters._

We have now passed in review, in more or less detail, the main facts on
which the theory of "the origin of species by means of natural
selection" is founded. In future chapters we shall have to deal mainly
with the application of the theory to explain the varied and complex
phenomena presented by the organic world; and, also, to discuss some of
the theories put forth by modern writers, either as being more
fundamental than that of Darwin or as supplementary to it. Before doing
this, however, it will be well briefly to summarise the facts and
arguments already set forth, because it