"Can you show us examples of how variation is NOT unlimited (outside of the organism *surviving*, of course)? What are the mechanisms that prohibit it from being unlimited? It's up to you to prove (or even SHOW) there's even one mechanism there, otherwise the variation just keeps on going and going - as evolution predicts and shows!"
Since evolutionists claim that common descent is a fact, demand that it is taught as fact, and are now even demanding that the word 'theory' be tossed out, the burden of proof is on you.
A wonderful way you could help your fellow evolutionists out is to use the criteria established by evolutionists (see http://whoisyourcreator.com/scientific_criteria.html ) and assess one piece of empirical evidence you find compelling that might prove common descent, then we could enter into an actual debate.
Second, the comment at the end of your post about 'quote mining' is interesting. Please site exactly what sentence(s) that the article proves common descent is true (by using empirical evidence).