Evolution in... cases has clearly been a very slow and almost (if not quite) continuous process, exactly as Darwin had predicted.
We must remember, however, that the organisms studied in this way are far from representative. They are in general the most successful members of animal associations living in very constant marine or lacustrine environments. We have not got similar data for land species... Nor do we possess them for the rarer forms. We shall see... that perhaps dominant species in a uniform environment are the least likely to undergo sudden change to a new type.

Introduction - The Causes of Evolution (1932)