What a species is has been debated for centuries but John Ray was the first person to produce a biological definition of what a species is. In his 1686 History of plants he states:
“..no surer criterion for determining species has occurred to me than the distinguishing features that perpetuate themselves in propagation from seed. Thus, no matter what variations occur in the individuals or the species, if they spring from the seed of one and the same plant, they are accidental variations and not such as to distinguish a species… Animals likewise that differ specifically preserve their distinct species permanently; one species never springs from the seed of another nor vice versa”
Remember that Ray did not know about genetics but recognized almost 200 years before Darwin and Mendel that like things come from like things and that those that give rise to like things represent a natural grouping which he recognized as species.