So that the islands of Bermuda and Madeira have been stocked from the neighbouring continents with birds, which for long ages have there struggled together, and have become mutually co-adapted.
Hence, when settled in their new homes, each kind will have been kept by the others to its proper place and habits, and will consequently have been but little liable to modification.
Any tendency to modification will also have been checked by intercrossing with the unmodified immigrants, often arriving from the mother-country.