MATERIALS OF HEREDITY 3 



a certain form — becomes a key that will unlock a door; 

 treated in another way it becomes a knife blade that will 

 cut. 



These two classes of differences are shown in a particu- 

 larly striking way by machines, and also by organisms. An 

 automobile or a typewriter depends in every detail of its 

 structure and its operation on the materials of which it is 

 made. By changing the materials of which the parts of the 

 machine are made, every feature of the machine can be 

 altered. Some materials make good parts, others poor ones. 

 But the structure and running of the machine depend too 

 on the way the materials are handled in the factory; depend 

 on the methods used in manufacturing the machine. The 

 same materials can be made into very different machines, 

 by changing the method of manufacture. 



Organisms, including human beings, show the same two 

 classes of differences. It has been discovered that in every 

 distinguishable property organisms depend on the ma- 

 terials of which they are composed at the beginning of 

 their lives; that is, on the materials that they receive from 

 their parents. Structure, physiology, mind, behavior — all 

 may be altered by changing these materials. Different In- 

 dividuals, in man and in other organisms, receive different 

 sets of materials from their parents. And it is found that 

 every feature or quality of the organism — physical, physi- 

 ological, or mental — can be changed by substituting one 

 kind of material for another. It is found that a great pro- 

 portion of the differences observed between different per- 

 sons, or between different animals or plants, are the result 

 of the different materials with which they begin Hfe. 



It is the differences that result from these different ma- 

 terials received from the parents at the beginning of life 

 that constitute the special subject matter of Genetics. 

 Genetics may be defined as an account of the effects of the 

 diversity of materials with which different individuals start 



