CONTENTS. vii 



difficulty Natura non facit saltum Organs of email 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 124 



CHAPTER VII. 



MlSpELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION. 



Longevity Modifications not necessarily simultaneous Modifications ap- 

 parently of no direct service Progressive development Characters of 

 small functional importance, the most constant Supposed incompetence 

 of natural selection to account for the incipient 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 166 



CHAPTER VIII. 



INSTINCT. 



Instincts comparable with habits, but different in their origin Instincts 

 graduated Aphides and ants Instincts variable Domestic instincts, 

 their oriin 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 191 



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 hybrid.* 

 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 

 mongrels compared independently of their fertility Summary . . 218 



b 



