atural selection we can clearly understand the full
meaning of that old canon in natural history, "Natura non facit saltum."
This canon, if we look only to the 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.

* * * * *

{207}

CHAPTER VII.

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 clas