Thomas Andrew Knight

“ The external form and character of each plant, as it came from the hand of nature, was probably sufficiently peculiar to render it readily distinguishable from those of every other species: but varieties of soil, of climate, and of culture applied for other purposes, have so far mixed and confounded the primary characteristics of many species, that experiments, such as those above described, now afford probably the only source of decisive evidence. ”

“ By a species of plants, I mean all plants which can be made to breed together without producing mules; that is, without producing plants which are incapable of affording offspring by seeds: and I consider all plants to be of distinct and different species which cannot be made to breed with each other (if capable of breeding at all) , or which, if they intermix, produce mule plants. ”