How do those groups of species, which constitute what are called distinctgenera, and which differ from each other more than do the species of the same genus, arise?

As species of the same genus have usually, though by no means invariably, some similarity in habits and constitution, and always in structure, the struggle will generally be more severe between species of the same genus, when they come into competition with each other, than between species of distinctgenera. We see this in the recent extension over parts of the United States of one species of swallow having caused the decrease of another species.