WHY LIKE BEGETS LIKE 15 



show that it was inherently rather improbable. Darwin, 

 cautious and thorough thinker as he was, saw this very 

 clearly ; yet he was forced to some such theory by what 

 he believed to be good evidence for the inheritance of 

 acquired characters. By acquired characters we mean 

 mutilations, modifications due to disease or to use or 

 disuse of special organs. 



It will be seen that if such characters are ever in- 

 herited, then we must assume some very intimate sort 

 of connection between the various parts of the body 

 and the reproductive organs some such connection as 

 Darwin assumed in his Pangenesis theory. 



We shall return later to this subject ; it has been, 

 perhaps, the most disputed question in the whole sub- 

 ject of heredity, and cannot be said to be yet settled. 

 It is enough to say, in the meantime, that the belief 

 in the inheritance of acquired characters was general 

 in Darwin's time, but is held only by a minority of 

 biologists to-day. 



The theory of heredity which is most widely accepted 

 at the present time, and which is, in fact, the only 

 theory that is of much service in helping us to under- 

 stand the facts, is the theory of the continuity of the 

 germ plasm. The development of this main idea has 

 been the great work of the German biologist Weismann. 

 In Weismann's own words, the idea is that part of the 

 germ plasm " contained in the parent egg-cell, is not 

 used up in the construction of the body of the offspring, 

 but is reserved unchanged for the formation of the germ 

 cells of the following generation." 



We may illustrate this diagrammatically, as in Fig. 2. 



The germ cell G a fertilised egg divides into a 

 number of cells, the group A, which constitute a new 

 individual. The white circles may be taken to represent 

 the cells of the specialised organs of the body, bones, 

 muscles, skin, and so forth. The germ cells of the 

 individual A are not formed from the body cells, but 

 are derived directly from the germ cells of the preceding 

 generation. The dark circles being taken to represent 

 the germ cells, it will be seen that the line of descent is 



