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 108 



CHAPTER VII. 



MISCELLANEOUS OBJECTIONS TO THE THEORY OP 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 — Reasons for disbelieving in great and abrupt 

 modifications 199 



CHAPTER VIIL 



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, 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 243 



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 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 univereal— Hybrids and mon- 

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



