OUR NATURAL INHERITANCE 59 



(except sterility) are heritable and may be handed on. 

 But ' may ' cannot be changed into * must/ for the 

 unexpected often happens. 



§ 4. Recent Advances in the Study of 

 Heredity 

 There are three modern ideas that have profoundly 

 influenced our views of heredity, (a) The first is the 

 idea of germinal continuity , which we owe especially to 

 Sir Francis Galton and Professor August Weismann . The 

 reason for like begetting like is to be found in the per- 

 sistence of a specific organisation through a lineage 

 of unspecialised germ-cells. The germinal material of 

 the fertihsed ovum forms the basis of the building 

 material out of which the body of the offspring is built 

 up, undergoing, in a puzzling way, not only a huge 

 increase in quantity but a qualitative differentiation 

 into nerve and muscle, blood and bone. But while 

 this is going on, a residue of the germinal material is 

 kept intact and unspecialised to form the beginning of 

 the reproductive organs of the offspring, whence may be 

 launched in due time another similar vessel on the 

 adventurous voyage of life. The sex-cells produced in 

 the reproductive organs are the descendants of unspe- 

 cialised embryonic cells, which did not share in body- 

 making, which did not become specialised. In short, 

 these germ-cells remain like the fertilised egg-cell from 

 which the organism started ; they continue the specific 

 tradition intact. As it has been put, instead of saying 

 that the hen gives rise to the egg, we should say the q^^ 



