CONTENTS. 



CHAPTER I. 



WHAT IS HEREDITY ? 



The development of an animal, with the complex and beauti- 

 ful structural adjustments, the instincts, habits, and in- 

 dividual traits of its parents is one of the most wonderful 

 phenomena of the material universe Heredity is not due 

 to the external conditions which act upon the ovum, but 

 to something within the ovum itself The phenomena of 

 reversion Asexual and sexual heredity Possibility of an 

 explanation of heredity Characteristics which are now 

 hereditary were at one time new variations Heredity and 

 variation are opposite aspects of the same problem We 

 may hope that a more perfect acquaintance with the laws 

 of heredity will remove many objections to the theory of 

 natural selection 5 



CHAPTER II. 



HISTORY OP THE THEORY OF HEREDITY. 



Requisites of a theory of heredity Historical sketch of specu- 

 lation on heredity Evolution hypothesis of Bonnet and 

 Haller Ovists and spermists Modern embryological re- 

 search has shown that it is impossible to accept the evolu- 

 tion hypothesis in its original form Buffon's speculations 

 upon heredity fails to account for variation Hypothesis 

 of epigenesis This hypothesis is logically incomplete 

 The analogy between pliylogeny and ontogeny gives no 

 real explanation of the properties of the ovum Haeckel's 

 plastidule hypothesis This hypothesis is not logically 

 complete unless it involves the idea of evolution Jager's 

 hypothesis Ultimate analysis shows that this is at bottom 

 an evolution hypothesis No hypothesis of epigenesis is 

 satisfactory No escape from some form of the evolution 

 hypothesis This conclusion is accepted by Huxley 16 



