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.

But in making these and the following remarks, I am compelled to allude to subjects hereafter to be discussed.

Look at the many outlying islands round a continent, and see how many of their inhabitants can be raised only to the rank of doubtful species.

So it is if we look to past times, and compare the species which have just passed away with those still living within the same areas; or if we compare the fossil species embedded in the sub-stages of the same geological formation.

It is indeed manifest that multitudes of species are related in the closest manner to other species that still exist, or have lately existed; and it will hardly be maintained that such species have been developed in an abrupt or sudden manner.