Darwin then presented the stronger evidence that human races are all the same species, noting that when races mixed together, they intercrossed beyond the "usual test of specific distinctness" and that characteristics identifying races were highly variable. He put great weight on the point that races graduate into each other, writing "But the most weighty of all the arguments against treating the races of man as distinct species, is that they graduate into each other, independently in many cases, as far as we can judge, of their having intercrossed", and concluded that the stronger evidence was that they were not different species.