42 THE PHYSICAL BASIS OF INHERITANCE 



heredity which is, on the whole, simpler — which seems, on the 

 whole, to fit the facts better, for instance the fact that our 

 experience does not warrant the conclusion that the modifica- 

 tions or acquired characters of the body of the parent affect 

 in any specific and representative way the inheritance of the 

 offspring. 



The Idea of Germinal Continuity. — As is well known, the view 

 which many, if not most, biologists now take of the uniqueness 

 of the germ-cells is rather different from that of pangenesis. It 

 is expressed in the phrase " germinal continuity," and has been 

 independently suggested by several biologists, though Weismann 

 has the credit of working it out into a theory. Let us state its 

 purport. There is a sense, as Galton says, 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. In many cases, 

 scattered through the animal kingdom, from worms to fishes, the 

 beginning of the lineage of germ-cells is demonstrable in very 

 early stages before the differentiation of the body-cells has more 

 than begun. In the development of the threadworm of the horse, 

 according to Boveri, the very first cleavage divides the fertilised 

 ovum into two cells, one of which is the ancestor of all the body- 

 cells, and the other the ancestor of all the germ-cells. In other 

 cases, particularly among plants, the segregation of germ-cells 

 is not demonstrable until a relatively late stage. Weismann, 

 generalising from cases where it seems to be visibly demonstrable, 

 maintains that in all cases the germinal material which starts 

 an offspring owes its virtue to being materially continuous with 

 the germinal material from which the parent or parents arose. 

 But it is not on a continuous lineage of recognisable germ-cells 

 that Weismann insists, for this is often unrecognisable, but on 

 the continuity of the germ-plasm — that is, of a specific substance 

 of definite chemical and molecular structure which is the bearer 



