CHAPTER III. 



HISTORY OF THE THEORY OF HEREDITY — {Coutimied). 



Some form of the evolution hypotliesis 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 — Difliculty 

 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 — AVe must therefore distrust its absolute correctness 

 — Summary of last two chapters. 



Some Form of tlie Evolution Hypothesis a Logical . 



Necessity. 



Most of the hypotheses which have hoen proposed, of 

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

 like the two we have qtioted, epigenesis hypothesis, for 

 they are attempts to show that the ovtim 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, hut 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- 



