This is how so many varieties came to populate the Earth. Sudden appearance was only an effect of many small changes over huge periods. Darwin did not explain how inherited variety arose and persisted. But he believed a complete fossil record would show many intermediate types. So new varieties can arise and will persist.
Genes allow variety to arise by mutation, and be passed on as distinct traits.
Small changes will accumulate as a continuous effect. Darwin's thesis of continuous small accumulation is shown to be consistent with latest models of gene flow. Fossil record still shows evolution mostly in bursts, with long periods of stability.