X CONTENTS. 



CHAPTER III. ' 



HISTORY OP 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 

 liybrids, from variation, and from structures confined to 

 one sex — Tlie 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 liypothesis 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 — There are raanj^ congenital characters which 

 are not hereditar}^ — Direct 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 — Tiiis 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 



