CONTENTS. xvu 



PAGB 



ment — Characters of small functional importance, the most 

 constant — Supposed incompetence of natural selection to 

 account for the incipiert stages of useful structures — Causes 

 which interfere with the acquisition through natural selection 

 of useful structures — Gradations of structure 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 ...... 187 



CHAPTER VIII. 



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 227 



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 hy- 

 brids — Parallelism between the effects of changed conditions 

 of life and of crossing — Dimorphism and Trimorphism — 

 Fertility of varieties when crossed and of their mongrel off- 

 spring not universal — Hybrids and mongrels compared inde- 

 pendently of their fertility — Summary 260 



CHAPTER X. 



ON THE IMPERFECTION OF THE GEOLOGICAL RECORD. 



On the absence ©f 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 denuda- 

 tion and of deposition — On the lapse of time as estimated by 

 years — On the poorness of our palaeontological collections — 

 On the intermittence of geological formations — On the denu- 

 dation of granitic areas — On the absence of intermediate 

 varieties in anyone formation — On the sudden appearance of 

 groups of species — On tb°*r sudden appearance in the lowest 

 known fossiliferous strata -Antiquitv of the habitable earth. 293 



