Later Additions up to the year 1892. 39 



repository of the material of heredity is merely hypo- 

 thetical. 



We now arrive at the last of those features in 

 Weismann's theory of heredity, the importance of 

 which necessitates mention in such a mere statement 

 of the theory as the present chapter is concerned 

 with. 



According to Weismann's own view of his theory, 

 two objections have to be met. In the first place, 

 there is the objection that all individuals which are 

 bom of the same parents are not exactly alike, as the 

 theory might have expected they would be, seeing that 

 the admixture of identical germ-plasms has been con- 

 cerned in the formation of the whole progeny. In 

 the second place, and quite apart from this objection, 

 there is the difficulty that, if every act of fertilization 

 essentially consists in a fusion of one mass of germ- 

 plasm belonging to a male germ-cell with another mass 

 belonging to a female germ-cell, at each generation 

 the mass of germ-plasm contained in an egg-cell 

 must be doubled — with the result that ova must 

 progressively increase in size during the course 

 of phylogeny. But ova do not thus progressively 

 increase in size. Therefore, if the imperishable nature 

 of germ-plasm is to be theoretically sustained, it 

 is necessary to show some means whereby ova 

 and spermatozoa are able to get rid of at least 

 one half of their respective germ-plasms in each 

 generation — i. e., before each act of impregnation. 

 Weismann meets both these difficulties by an appeal 

 to the following facts. 



It is well known that the ripe ovum extrudes two 



