 They had six children.In his work as a natural historian, he described a great number of new species of small mammal on the islands around the British Isles, notably the house mice and field mice of St. Kilda which he called "Mus muralis" and "Mus hirtensis", believing that these had evolved "in situ" having colonised the islands naturally via land or ice-bridges.Although this has been demonstrated to be wrong, and many of his described species are now regarded as island forms rather than species in their own right, his contribution to natural history was enormous.