Can only alter what already exists. They cannot alter what does not exist or explain how it came into existence. The vast majority of mutations are deleterious or neutral at best. The changes or variations observed in living populations are always limited. The change can go only so far and then ceases.
All organisms seem to be able to vary around an average type, but never far from the average.
Artificial selection, i.e., selective breeding by man of new varieties of plants and animals likewise reveals the same tendency for limited variation only. Cats are still cats, dogs still dogs, roses still roses. Large accumulated change leads to sterility. The theory of evolution is built on the assumption that the limited changes that have been observed in nature, continued for millions of years, could indeed transform primeval slime into modern people. This kind of thinking is called extrapolation.