present inhabitants of the world, is
not strictly correct, but if we include all those of past times, it must
by my theory be strictly true.

It is generally acknowledged that all organic beings have been formed on
two great laws--Unity of Type, and the Conditions of Existence. By unity
of type is meant that fundamental agreement in structure, which we see
in organic beings of the same class, and which is quite independent of
their habits of life. On my theory, unity of type is explained by unity
of descent. The expression of conditions of existence, so often insisted
on by the illustrious Cuvier, is fully embraced by the principle of
natural selection. For natural selection acts by either now adapting the
varying parts of each being to its organic and inorganic conditions of
life; or by having adapted them during long-past periods of time: the
adaptations being aided in some cases by use and disuse, being slightly
affected by the direct action of the external conditions of life, and
being in all cases subjected to the several laws of growth. Hence, in
fact, the law of the Conditions of Existence is the higher law; as it
includes, through the inheritance of former adaptations, that of Unity
of Type.

7. INSTINCT.

Instincts comparable with habits, but different in their origin.
Instincts graduated. Aphides and ants. Instincts variable. Domestic
instincts, their origin. Natural instincts of the cuckoo, ostrich, and
parasitic bees. Slave-making ants. Hive-bee, its cell-making instinct.
Difficulties on the theory of the Natural Selection of instincts. Neuter
or sterile insects. Summary.

The subject of instinct might have been worked into the previous
chapters; but I have thought that it would be more convenient to treat
the subject separately, especially as so wonderful an instinct as that
of the hive-bee making its cells will probably have occurred to many
readers, as a difficulty sufficient to overthrow my whole theory. I must
premise, that I have nothing to do with the origin of the primary mental
powers, any more than I have with that of life itself. We are concerned
only with the diversities of instinct and of the other mental qualities
of animals within the same class.

I will not attempt any definition of instinct. It would be easy to show
that several distinct mental actions are commonly embraced by this term;
but every one understands what is meant, when it is said that instinct
impels the cuckoo to m