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.