As well as Darwin, the Reverend J.T. Gulick also found Wagner's theories overstated., but in a later paper Gulick says that "Moritz Wagner, in his "Law of the Migration of Organisms", was the first to insist on the importance of geographical isolation as a factor in evolution, but when he asserted that, without geographical isolation, natural selection could have no effect in producing new species, he went beyond what could be sustained by facts".