ich serve as a
sign of inedibility and prevent their being needlessly attacked. This,
however, is a portion of the very large subject of organic colour and
marking, which will be fully discussed and illustrated in a separate
chapter.

In this way every possible modification of an animal or plant, whether
in colour, form, structure, or habits, which would be serviceable to it
or to its progeny at any period of its existence, may be readily brought
about. There are some curious organs which are used only once in a
creature's life, but which are yet essential to its existence, and thus
have very much the appearance of design by an intelligent designer. Such
are, the great jaws possessed by some insects, used exclusively for
opening the cocoon, and the hard tip to the beak of unhatched birds used
for breaking the eggshell. The increase in thickness or hardness of the
cocoons or the eggs being useful for protection against enemies or to
avoid accidents, it is probable that the change has been very gradual,
because it would be constantly checked by the necessity for a
corresponding change in the young insects or birds enabling them to
overcome the additional obstacle of a tougher cocoon or a harder
eggshell. As we have seen, however, that every part of the organism
appears to be varying independently, at the same time, though to
different amounts, there seems no reason to believe that the necessity
for two or more coincident variations would prevent the required change
from taking place.

_The Continued Existence of Low Forms of Life._

Since species are continually undergoing modifications giving them some
superiority over other species or enabling them to occupy fresh places
in nature, it may be asked--Why do any low forms continue to exist? Why
have they not long since been improved and developed into higher forms?
The answer, probably, is, that these low forms occupy places in nature
which cannot be filled by higher forms, and that they have few or no
competitors; they therefore continue to exist. Thus, earthworms are
adapted to their mode of life better than they would be if more highly
organised. So, in the ocean, the minute foraminifera and infusoria, and
the larger sponges and corals, occupy places which more highly developed
creatures could not fill. They form, as it were, the base of the great
structure of animal life, on which the next higher forms rest; and
though in the course of ages they may undergo some c