That many species have been evolved in an extremely gradual manner, there can hardly be a doubt.
The species and even the genera of many large natural families are so closely allied together that it is difficult to distinguish not a few of them.
On every continent, in proceeding from north to south, from lowland to upland, etc.
, we meet with a host of closely related or representative species; as we likewise do on certain distinct continents, which we have reason to believe were formerly connected.