HEREDITY 87 



of the nucleus with the material substance on which in- 

 heritance depends for the following, among other, reasons. 



(1) Equality in quantity of chromatin in the two 

 gametes, in spite of their enormous disparity in size as 

 a whole. 



(2) Accurate division of each element of this material 

 at each nuclear division, so that every cell in the body 

 gets a derivative of all the chromatin elements of both 

 gametes. 



(3) Reduction of the amount of chromatin to one half 

 in the gametes, so that doubling is avoided when they 

 unite. 



With this knowledge of the physical basis of inheri- 

 tance, we must proceed to consider the organism as 

 forming one of an endless series of generations. 



Perhaps the easiest way to grasp the point of view 

 from which modern biologists regard the organism as a 

 link in the chain of ascendants and descendants is to com- 

 pare the two theories of heredity propounded, the one by 

 Darwin and the other by Gait on. With fuller knowledge, 

 Darwin's hypothesis has had to be rejected. (It must not 

 of course be supposed that we are referring in any way 

 to his great work on Evolution by Natural Selection.) 



Darwin proceeded from what may be called the natural 

 or common sense way of looking at the organism. He 

 and his contemporaries were exercised as to how the 

 characters of the parent got into the gamete. In break- 

 ing into the endless chain of parent-germ-parent (the old 

 problem : which came first, the chicken or the egg ?) he 

 started with the parent, and the problem of heredity 

 was : how could its characteristics be compressed into the 

 minute and apparently nearly structureless gamete? 



