CONTENTS. 
VH 
CHAPTEE VI. 
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 .Page 171-206 
CHAPTEE VIL 
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 
207-244 
CHAPTEE VIII. 
Hybridism. 
Distinction between the sterility of first crosses and of hybrids — 
Sterility various in degree, not universal, affected by close inter¬ 
breeding, 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 con¬ 
ditions 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 
245-278 
