CONTENTS. xix 



Page. 

 small importance — Organs not in all cases absolutely per- 

 fect — The law of Unity of Type and of the Conditions of 

 Existence embraced by the theory of Natural Selection 158 



CHAPTER VII; 



MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL 

 SELECTION. 



Longevity — Modifications not necessarily simultaneous — Modi- 

 fications apparently of no direct service — Progressive 

 development — Characters of small functional importance, 

 the most constant — Supposed incompetence of natural selec 

 tion to account for the incipient stages of useful structures 

 — Causes which interfere with the acquisition through 

 natural selection of useful structures — Gradations of struct- 

 ure with changed functions — Widely different organs in 

 members of the same class, developed from one and the 

 same source — Heasons for disbelieving in great and abrupt 

 modifications 199 



CHAPTER VIII. 



INSTINCT. 



istincts 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 of the theory of the Natural Selection of instincts 

 — Neuter or sterile insects — Summary 242 



CHAPTER IX. 



HYBBIDISM. 



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 accumu- 

 lated 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 mon- 

 grels compared independently of their fertility— Summary.. .277 



