X CONTENTS. 



CHAPTER III. 



HISTORY OF THE THEORY OF HEREDITY (Continued). 



Some form of the evolution hypothesis a logical necessity 

 Darwin's pangenesis hypothesis This is an evolution 

 hypothesis, since all the characteristics of the adult are 

 supposed to be latent in the germ Miscellaneous objec- 

 tions to it These objections do not show that it conflicts 

 with fact Difficulty in imagining detailed working is no 

 reason for rejecting it-^Galton's experimental disproof 

 There are many reasons for believing that the sexual ele- 

 ments have different functions The evidence from par- 

 thenogenesis Polar-cell hypothesis The evidence from 

 hybrids, from variation, and from structures confined to 

 one sex The pangenesis hypothesis recognizes no such 

 difference in the functions of the reproductive elements 

 We must therefore distrust its absolute correctness Sum- 

 mary of last two chapters 47 



CHAPTER IV. 



A NEW THEORY OF HEREDITY. 



The objection to the hypothesis of pangenesis would be 

 almost entirely removed if it could be simplified State- 

 ment of a new theory Heredity is due to the properties 

 of the egg^Each new character has been impressed upon 

 the egg by the transmission of gemmules -Tendency to 

 form gemmules is due to the direct action of external con- 

 ditions The ovum is the conservative element The male 

 cell is the progressive element This theory has features 

 of resemblance to most of the hypotheses which have been 

 noticed It fills most of Mivart's conditions also It is not 

 necessary to assume that the ovum is as complicated as 

 the adult There are many race characters which are not 

 congenital-^rThere are many congenital characters which 

 are not hereditary-4Direct action of external conditions 

 Our theory stands midway between Darwin's theory of 

 natural selection and Lamarckianism 80 



CHAPTER V. 



ON THE OPINION THAT EACH SEX MAY TRANSMIT ANY 

 CHARACTERISTIC WHATEVER. 



The argument from hybrids This argument is inconclusive 

 The argument from the homology between the ovum 

 and the male cell Homology does not involve functional 

 similarity The argument from the dual personality of 

 each individual; from reversion ; and from polymorphism 

 These phenomena admit of a simpler explanation 

 Summary of chapter 99 



