No, it has not stopped. But you are talking about a process that is incredibly slow. Evolution happens through random mutations at birth. A snake isn't going to just be slithering along one day and suddenly start to grow a leg as you mentioned. Most random mutations are harmful to the particular creature that is born with it and do not enhance it's ability to survive. Every once in a very rare while a creature is born with a random mutation that actually helps it survive better than the others of it's speices. Animals that survive better live longer to spread their genes to offspring that will also have the mutation or at least the potential for it. But there may be hundreds of thousands of life cycles in a certain species before such a beneficial mutation occurs if ever. And even then it may take a very long time for the decendents of that original genetic mutation to increase in numbers until they become the new normal for that species. We humans live about 80 years or so. That being the case it's pretty hard for us to observe a process as slow as evolution.