habits
widely different from those of their allies--Organs of extreme
perfection--Modes of transition--Cases of difficulty--Natura non facit
saltum--Organs of small importance--Organs not in all cases absolutely
perfect--The law of Unity of Type and of the Conditions of Existence
embraced by the theory of Natural Selection.

CHAPTER VII.

MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.

Longevity--Modifications not necessarily simultaneous--Modifications
apparently of no direct service--Progressive development--Characters of
small functional importance, the most constant--Supposed incompetence
of natural selection to account for the incipient stages of useful
structures--Causes which interfere with the acquisition through natural
selection of useful structures--Gradations of structure with changed
functions--Widely different organs in members of the same class,
developed from one and the same source--Reasons for disbelieving in
great and abrupt modifications.

CHAPTER VIII.

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, molothrus, ostrich, and parasitic bees--Slave-making
ants--Hive-bee, its cell-making instinct--Changes of instinct and
structure not necessarily simultaneous--Difficulties on the theory of
the Natural Selection of instincts--Neuter or sterile insects--Summary.

CHAPTER IX.

HYBRIDISM.

Distinction between the sterility of first crosses and of
hybrids--Sterility various in degree, not universal, affected by close
interbreeding, removed by domestication--Laws governing the sterility
of hybrids--Sterility not a special endowment, but incidental on
other differences, not accumulated by natural selection--Causes of
the sterility of first crosses and of hybrids--Parallelism between the
effects of changed conditions of life and of crossing--Dimorphism and
Trimorphism--Fertility of varieties when crossed and of their mongrel
offspring not universal--Hybrids and mongrels compared independently of
their fertility--Summary.

CHAPTER X.

ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.

On the absence of intermediate varieties at the present day--On the
nature of extinct intermediate varieties; on their number--On the lapse
of time, as inferred f