CHAPTER III. 



^HISTORY OF THE THEORY OF HEREDITY (Continued). 



Some form of the evolution hypothesis a logical necessity 

 Darwin's paugenesis hypothesis This is an evolution hypothe- 

 sis, since all the characteristics of the adult are supposed to be 

 latent in the germ Miscellaneous objections to it These 

 objections do not show that it conflicts with fact Difficulty 

 in imagining detailed working is no reason for rejecting it 

 Gallon's experimental disproof There are many reasons 

 for believing that the sexual elements have different functions 

 The evidence from parthenogenesis Polar-cell hypothesis 

 The evidence from hybrids, from variation, and from struc- 

 tures confined to one sex The pangenesis hypothesis recog- 

 nizes no such difference in the functions of the reproductive 

 elements We must therefore distrust its absolute correctness 

 Summary of last two chapters. 



Some Form of the Evolution Hypothesis a Logical 

 Necessity. 



Most of the hypotheses which have been proposed, of 

 late years, to account for the phenomena of heredity, are 

 like the two we have quoted, epigenesis hypothesis, for 

 they are attempts to show that the ovum is in reality, as 

 well as in form, an nnspecialized cell. Analysis shows, 

 however, that they all rest ultimately upon the assump- 

 tion that this is not true, but that the ovum really con- 

 tains, in some form or other, actually or potentially, the 

 future organism, with all its hereditary characteristics. 



We know that eggs which are to all appearances essen- 

 tially alike, may, when artificially removed from the ova- 



