496 ZOOLOGY 



tends to be not about the extreme parent, but rather about the 

 mean of the group. The offspring of the mutants, on the 

 other hand, have their mean about the parent, and are quite 

 independent of the general mean. Mutations are not necessarily- 

 discontinuous nor extreme variations, however. Small muta- 

 tions behave as described above. In other words mutations are 

 transmissible; fluctuations are not. 



491. Heredity, — the Organic Continuity between Genera- 

 tions. — We have already seen (§131) that the germ cells, ovum 

 and sperm, are the organic bridge which connects the body of one 

 generation with that of the next. Just as new qualities cannot 

 influence evolution until they are represented in the protoplasm 

 of the germ cells (germ plasm) , so all the old qualities of the race, 

 both deep and superficial, in order to continue, must be repre- 

 sented in this germ plasm. We know, just because offspring 

 continue in the main like their parents, that this germ plasm 

 does not change easily ; but, because they are never exactly like 

 their parents, it does change some. When the germ cells have 

 united, inheritance in the new generation has been determined. 

 After this the parent may influence the development of the new 

 individuals, — as the mother mammal doubtless does while the 

 young is carried in the uterus or nourished by milk; but neither 

 of these forms of parental influence is a matter of heredity in any 

 sense. 



In those organisms in which there is no union, where a single 

 germ cell may develop into the adult, only one line of germ plasm 

 is involved and heredity seems a somewhat simpler thing. How- 

 ever, when two different strains of germ cells unite to start the 

 new generation, every cell of the new individual receives chromo- 

 somes, and probably other material, that came by way of both 

 ovum and sperm. We are very confident that certain hereditary 

 qualities are carried in some way by these chromosomes. Chro- 

 mosomes from the sperm,_ carrying definite characteristics of the 

 father's line, are closely mixed in the same nucleus with chromo- 

 somes of the egg carrying characteristics of the mother's line. 

 These maternal and paternal chromosomes are distributed 

 equally both to the body cells of the new individual and to the 



