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 from the rate of denudation and of deposition -- On