Theory of evolution

My thoughts about evolution: How are really new species appear? Suppose through series of mutation a "crocodile" was born from human. In order to survive and evolve he should share his genes with the other similar species. But there is no human species willing to share genes with "crocodile". So single mutation could change a little creature but could not produce a new class. There is big mutation needed in multiple species all together. I believe that super virus that spread across is the dominant source of change, If bunch of species have changed genes together, then a new born. So there should be some viruses that periodically emerge and penetrate a cell. Cell even may have a mechanism to understand what viruses or part of the virus it may accept to get beneficial state. Some kind low "potential energy" of cell. Some cells like nautilus is already in low energy state and because of this they do not evolve for a long period of time. The other like humans constantly accept mutations in search for batter state.