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- 
