¶ Look into the mirror the next time you are angry, happy, surprised, tired or sorrowful and note the changes wrought by your emotions in your facial muscles.
Constant repetition of the same kinds of thoughts or emotions finally makes permanent changes in that part of the body which is physiologically related to these mental processes.
The Evolution of the Jaw
¶ The jaw is a good illustration of this alliance between the mind and the body. Its muscles and bones are so closely allied to the pugnacity instinct center in the brain that the slightest thought of combat causes the jaw muscles to stiffen. Let the thought of any actual physical encounter go through your mind and your jaw bone will automatically move upward and outward.
After a lifetime of combat, whether by fists or words, the jaw sets permanently a little more upward and outward—a little more like that of the bulldog. It keeps to this combative mold, "because," says Mother Nature, the great efficiency expert, "if you are going to call on me constantly to stiffen that jaw I'll fix it so it will stay that way and save myself the trouble."
Inheritance of Acquired Traits
¶ Thus the more combative jaw, having become permanent in the man's organism, can be passed on to his children.
¶ Right here comes a most interesting law and one that has made possible the science of Human Analysis:
Law of Size
¶ The larger any part or organ the better its equipment for carrying out the work of that organ and the more does it tend to express itself. Nature IS an efficiency expert and doesn't give you an oversupply of anything without demanding that you use it.