There have to be many tiny mutations in order to get two different species. And the mutation/recombination of genes has to be 'useful' for the animal (in fact, every organism. So plants also evolve) in the environment in which it is living. Then the animal has perhaps a little bit more chance to produce offspring and to pass on his genes (and as a consequence to pass on his mutation).
Furthermore, isolation is also a condition to a new species. If the mutated genes are still being exchanged with the old ones, you aren't going to get two species. Only when two populations can evolve separately (in a different environment, so the definition of 'useful' is different for both), two new species will come to exist.
But yes, chances to get mutations, to get isolated, ... are very small (maybe not that small (one in a trillion), but I'm quite sure you can look up those figures). Chances to a 'useful' mutation are even smaller, but remember, they are selected.
For those who wouldn't see, I do believe in evolution. And I'm not really religious, but I wouldn't see why 'God' (or whoever you believe in) would be to 'blame' if he didn't create all organisms or even why this contests the existence of God. Evolution is just another wonder of nature to me...