The Primary Hereditary Characters 171 



into packets is fundamental for an understanding of how 

 heredity occurs ; it is this that directly brings about all the 

 peculiar phenomena that are called Mendelian inheritance. 

 And this is a typical illustration of the part played by 

 structure and arrangement in organisms ; it demonstrates 

 that a chemical study alone, omitting arrangement of the 

 chemicals, can never lead to comprehension of what occurs. 

 The point is that when two or more chemicals are in a cer- 

 tain space, it makes all the difference in the world as to 

 what happens, whether the two substances are in separate 

 bottles, or merely poured together. To neglect this fact 

 in organisms is as fatal to understanding them as it would 

 be to try to comprehend what occurs in a chemical labor- 

 atory without realizing that the different chemicals are 

 kept in separate containers. The nucleus of the cell is a 

 chemical laboratory containing diverse chemicals in separate 

 packets. At times substances come out of these packets, 

 intermingle, and therefore react. It is their reactions with 

 each other, and with external conditions, in an orderly way, 

 that bring about growth and the development into' a struc- 

 ture with diverse organs. The packets are shifted about and 

 distributed in certain ways at the time of mating and fer- 

 tilization, and it is the rules of their distribution that are 

 what we call the rules or laws of inheritance. 



We know that in any organism these packets of chemicals 

 are present in a definite arrangement. We know that each 

 larger packet or chromosome is a chain or group of con- 

 nected small packets (Figure 29, E, F), and that the dif- 

 ferent chromosomes present in a nucleus are diverse. Their 

 number is definite in any individual, and they are so con- 

 stituted as to form a set of diverse pairs (Figure 29; Figure 

 45 B); (though sometimes there is a single package or 

 chromosome that is not paired with another). 



