142 HEREDITY [AI\ 



somes must occur, either by elimination of complete 

 chromosomes or by transverse instead of longitudinal 

 splitting, before any complete observations had been 

 made showing that this actually happens. 



Since Weismann supposes that the germ-plasm 

 is contained in the chromosomes of the germ-cells, 

 and since half the chromosomes are removed in the 

 ' maturation' of these cells without preventing the 

 transmission of any part to the offspring by inherit- 

 ance, he concluded that each chromosome contains 

 all the units (' determinants ') necessary to a complete 

 individual. (Later work has rendered this conclusion 

 doubtful : see Appendix n.) When fusion of male 

 and female sex-cells takesplace, the resulting individual 

 will contain a mixture of the parental germ-plasms, 

 the paternal in some chromosomes, the maternal in 

 others. In the maturation of the sex-cells half 

 these germ-plasms will be removed and in the next 

 generation a fresh mixture will take place. It thus 

 follows that the different chromosomes contain germ- 

 plasms descended from different ancestors, and 

 different mixtures of these will occur in different 

 individuals. Here then we come to Weismann's 

 hypothesis of the origin of variation. Since different 

 individuals contain different combinations of an- 

 cestral germ-plasms, these will lead to varying 

 effects in the development of the body ; new com- 

 binations will be continually occurring in every 



