In his 1894 book, "Materials for the study of variation", Bateson took this survey of biological variation significantly further. He was concerned to show that biological variation exists both continuously, for some characters, and discontinuously for others, and coined the terms "meristic" and "substantive" for the two types. In common with Darwin, he felt that quantitative characters could not easily be "perfected" by the selective force of evolution, because of the perceived problem of the "swamping effect of intercrossing", but proposed that discontinuously varying characters could.