Which training principle states that exercising a certain body part or practicing a particular skill primarily develops that part or skill?

The training principle of Specificity states that exercising a certain body part or practicing a particular skill primary develops that part or skill. Ensuring that exercise programs have specific goals would make training a lot easier and improvements more noticeable. 

If you want to be a good runner, you have to incorporate running into your training program. If you want to beat that time in swimming, you have to incorporate sprints in your swim training. You are not only training your muscles for the activity, you also need your brain to be trained enough for the exact activity or skill that you are going to perform. Specificity principle reminds and ensures an individual to assign activities that would help meet specific goals.