THE THEORIES OF HEREDITY 09 



and any one who conies to consider, tor instance, the 

 means by which the ''gemmules" are to tind their 

 way to the reproductive glands, will agree that this 

 theory offers inherent ditficulties of the most formid- 

 able character. 



The second theory of heredity is expressed in the 

 fascinating conception of Weismann — " the con- 

 tinuity of the germ-plasm." This has already been 

 discussed. Here we merely note that it supplies a 

 most satisfactory explanation of the fact of heredity ; 

 and that it markedly contrasts with the preceding 

 theory in its bearing upon the belief in the transmis- 

 sion of acquirements. We may also note some remark- 

 able fashions in which the theory may be expressed. 

 If it be true, there is a sense, as Mr. Galton observes, 

 in which the child is as old as the parent, for when 

 the parent's body is developing from the fertilised 

 ovum, a residue of unaltered germinal material is 

 kept apart to form the future reproductive cells, 

 one of which may become the starting-point of a 

 child. Further, the theory gives a new meaning to 

 the saying that the child is " a chip of the old 

 block." As Thomson says, " Similar material to 

 start with, similar conditions in which to develop, 

 therefore, like tends to beget like." Lastly, note, 

 that on this theory the daughter is really the sister 

 of her mother. 



The third and most recent theory of heredity is 

 that of Oscar Hertwig. He denies the existence of 

 a cardinal distinction between the germ-plasm and 

 the plasm that goes to form the individual body. 

 He observes that in the higher plants there is wide- 

 spread occurrence of tissues which have the power 



