26 PROBLEMS OF EVOLUTION 



cells ; they contain germ-plasm unaltered, though they have not 

 the secondary characteristic of sex. The hydra, the remarkable 

 small creatures so often found clinging to duckweed, brings this 

 difficulty to a head. 



It habitually propagates by buds, and these buds break off 

 and grow into independent hydras in which both male and 

 female reproductive cells are developed ; the former assume 

 a tadpole form and swim to the egg cells. This shows clearly 

 that the germ-plasm is not isolated from the first. Weismann 

 has, however, proved that it is only from the ectoderm or outer 

 layer of the body that the reproductive cells are formed. 

 Hence, even in the hydra, there is some localisation. Dropping 

 Weismann's elaborate architectural terms ids, determinants, 

 biophors I think the facts may be provisionally explained 

 thus. The germ-plasm within the germ cells remains ab- 

 solutely unspecialised, it contains all the elements of which 

 the body is formed. But in some parts of the body there 

 is less specialisation than in others ; in the hydra and other 

 low organisms some of the body cells are (except for secondary 

 characters without which sexual generation would be impossible) 

 reproductive cells. In the higher animals specialisation has gone 

 much further and we may hold that in them there is a complete 

 localisation of the germ-plasm. As Mr Galton has put it, 

 " The main line may be rudely likened to the chain of a 

 necklace and the personalities to pendants attached to its links." 1 

 The same thing has been expressed in legal phraseology : the 

 relation of a parent to his child is that of trustee rather than 

 of testator. Thus we return to what was said above : the hen 

 is not the mother of the egg, but the egg is the mother of 

 the hen and of other eggs which may in due course develop 

 into chickens. 



(arwin's If we throw over the continuity of the germ-plasm, what 



>1S are the alternatives ? One is to adopt some such theory as 



Darwin's Pangenesis according to which the various parts of the 



body throw off innumerable minute particles, called by him 



1 Natural Inhtritance, p. 19. 



