CONTENTS. Vll 



CHAPTER 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 — 

 Xatura 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 



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 



207-244 



CHAPTER 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 



