micro-organisms vs higher animals

There is certain things that comes to mind while discussing on different life forms that prevail in the face of earth, well frankly speaking everyone knows about micro-organism from single celled bacteria to multi cellular higher animals. But the question comes which is the most stable living being if we try to compare them by the idea of Darwin or Lamarck, that is survival of the fittest.Well from my point of view mammals specially Human Beings and other higher plant and animals consists of various complex mechanisms to survive that too involves special systems to carry on their different body functions like digestive system, reproductive system, circulatory system, excretory system and so on. But the physiology of a microorganism is not so complex they are much more simple and hence stable, the micro-organism undergoes cell division like binary fission that too much faster reproductive process in compare to a higher animal and that produces millions of progeny withing a mere 5-6 hours. Thats the reason the micro-organism are the only living being theoretically which can pass the survival of the fittest exam much more easily in compare to a Human.