CHAPTER 6.  DIFFICULTIES ON THEORY.

Difficulties on the theory of descent with modification. 
Transitions. 
Absence or rarity of transitional varieties. 
Transitions in habits of life. 
Diversified habits in the same species. 
Species with habits widely different from those of their allies. 
Organs of extreme perfection. 
Means 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 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.

CHAPTER 8.  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.  Causes of the sterility of first crosses and of hybrids.  Parallelism between the effects of changed conditions of life and crossing.  Fertility of varieties when crossed and of their mongrel offspring not universal.  Hybrids and mongrels compared independently of their fertility.  Summary.

CHAPTER 9.  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 vast lapse of time, as inferred from the rate of deposition and of denudation.  On the poorness of our palaeontological collections.  On the intermittence of geological formations.  On the absence of intermediate varieties in any one formation.  On the sudden appearance of groups of species.  On their sudden appearance in the lowest known fossiliferous strata.

CHAPTER 10.  ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.

On the slow and successive appearance of new species. 
On their different rates of change. 
Species once lost do not reappear. 
Groups of species follow the same general rules in their appearance
and disappearance as do single species. 
On Extinction. 
On simultaneous changes in the forms of life throughout the world. 
On the affinities of extinct species to each other and to living
species. 
On the state of development of ancient forms. 
On the succession of the same types within the same areas. 
Summary of preceding and present chapters.