Don't believe me? Then use yourself as an experiment. If you're super skinny and can't do one pull-up, then watch what would happen when you're able to do 20 pull-ups. There is no way that your body won't get more muscular. Or, if you're only able to bench-press 60 lbs, there is no way your body could remain the same if you've progressed to benching 300 lbs.
Now, not everybody is going to look exactly the same. The skinny guy won't look as hulky as someone who naturally has a wider frame or more muscle mass. However, BOTH will see major muscular growth as long as they are consistent with their program and frequently progress by challenging themselves.
When you challenge yourself, you give your muscles reasons to grow. When you're consistent, you'd be sure that you WILL see results within a relatively short time. Those who aren't consistent would take months, even years, to see growth. Those who are consistent are the ones seeing results within a matter of weeks if not months!
I take myself as an example. Back in college, I was able to lift 70 lbs. Fast forward that 10 years later. I still found myself ONLY lifting 70 lbs. Needless to say, I was only SLIGHTLY more toned than I was back in my college days! What's up with that??? I'll tell you: inconsistency.
No progression was involved because I wasn't even being consistent. In order for progression to work, you need to be consistent. How could I move on to lifting 80 lbs if I was only able to lift 70 lbs? How could I do one-arm push-ups if I could barely do regular push-ups? If you aren't consistent, you can't progress, and no progression means no results.
So, don't look for the fastest, easiest way to gain muscles. There are no secrets to building muscle. Just be consistent and progress. You WILL see results guaranteed!