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 OF 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 fail to account for variation — Hypothesis 

 of epigenesis — This hypothesis is logically incomplete — 

 The analogy between phylogeny 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 



