238 THE MEANING OF EVOLUTION 



no inheritance of acquired characters." The biologi- 

 cal world has had no shock equal to this since Dar- 

 win's time, and there are few other questions to which 

 scientists to-day return with such constant vigor. 



If what Weissman says is true, that no variation 

 or development which comes to an animal during his 

 lifetime can be transferred into his own germ cells 

 and handed on to his children, then it becomes evi- 

 dent that we must find some cause of variation that 

 acts within the germ cells. This is the difficulty 

 which Weissman meets. He says that there are small 

 particles in the nucleus of each cell ; that these par- 

 ticles which he calls determinants decide the form and 

 the course of development of that cell ; that when that 

 cell divides to produce another cell it gives to this 

 other cell one-half of each determinant. As a result 

 the second cell grows to be like the first. This tells us 

 why offspring are like their parents. There is noth- 

 ing in the theory thus far to show us why offspring 

 are not exactly like their parents. In other words, 

 there is no accounting, thus far in the theory, for 

 variation. When the biologist studies carefully the 

 history of an egg while it is being formed, he sees 

 that at one stage in its development it throws away 

 not one-half of each determinant, but one-half of the 

 determinants. When an egg does this, it deliberately 

 casts aside one-half of the possibilities of its own de- 



