Mr.Hutchinson has given us some excellent English data to begin with.
Then I would follow this up by contrasting the various parallel forms of life in the two continents.

Our naturalists have often referred to this incidentally or expressly; but the animus of Nature in the two half globes of the planet is so momentous a point of interest to our race, that it should be made a subject of express and elaborate study.

Go out with me into that walk which we call THE MALL, and look at the English and American elms.

The American elm is tall, graceful, slender-sprayed, and drooping as if from languor.

The English elm is compact, robust, holds its branches up, and carries its leaves for weeks longer than our own native tree.
Is this typical of the creative force on the two sides of the ocean, or not?
Nothing but a careful comparison through the whole realm of life can answer this question.
There is a parallelism without identity in the animal and vegetable life of the two continents, which favors the task of comparison in an extraordinary manner.