314 Heredity. 



definite, restricted functions exactly and efFicientlv, 

 while eacli part of a low organism fills many ofidces, but 

 fills them all imperfecll}^ 



We find in all excei)t the lowest organisms that he- 

 redity is brought about by two dissimilar reproductive 

 elements, and we find that each organism is the resultant 

 of two factors — heredity and variation. 



It is natural to inquire whether there may not be some 

 connection between these two relations; whether the 

 natural selection of favorable variations has not acted 

 upon the reproductive elements as it has upon the mature 

 organisms; whether it has not brought about a physiolog- 

 ical division of labor between these elements; whether 

 their originally similar functions have not gradually 

 become s])ecialized until one has become the conservative 

 medium, and theotherthe agent of progress in heredity. 



According to the view advocated in this book, such 

 has actually been the history of the evolution of sex, 

 and natural selection has evolved, in all the higher 

 organisms, a secondary law of heredity, which enables it 

 to do its work rapidly and effectively, with little waste. 



In the metazoa and in the higher plants, natural 

 selection is not a crude, rough ^' hit or miss" method of 

 evolution, for the law of heredity, itself a result of the 

 law of natural selection, is that the ovum is the vehicle 

 of heredity, while gemmules or cell germs from cells in all 

 parts of the body, are transmitted to the ovum by the 

 male cell, thus causing variation when and where it is 

 needed. 



This view is opposed to the conclusion of many high 

 authorities that there is no difference in the functions of 

 the sexual elements, but examination shows that the 

 reasons which they have given for this conclusion admit 

 of another cimple explanation. 



