HEREDITY, VARIATION AND DEATH 25 



multiply by fission : and we can understand why each is like all, 

 for they are all parts of the same. If we adopt the theory of 

 the continuity of the germ-plasm then we account for the 

 likeness of father and son on the same principle as we account 

 for the likeness of one amoeba to another. 



Lastly, there is some direct evidence. In the Diptera, 1 as 

 different observers have discovered independently, the repro- 

 ductive cells arise directly from the parent egg cell and are 

 set apart from the first. Here, there is proved continuity. 

 In leeches and some arthropods they are set apart very early. 



FIG. 8. A, Hydra ; B, Sperm or male cell of hydra ; nttk, mouth ; 

 spy, two spermaries ; ozy, an ovary. From Parker's Biology. 



In other classes the separation is put off for some days or 

 weeks and this is, no doubt, a difficulty. But, I believe the 

 difficulty is slight. In these we may hold that at least some 

 of the body cells are up to this time potentially reproductive 



1 See Weismann : Essays on Heredity, vol. i. p. zoo. 



