A species consists of a group of populations which replace each other geographically or ecologically and of which the neighboring ones integrate or hybridise wherever they are in contact or which are potentially capable of doing so (with one or more of the populations) in those cases where contact is prevented by geographical or ecological barriers.

Systematics and the Origin of Species, from the Viewpoint of a Zoologist (ed. Harvard University Press, 1942) - ISBN: 9780674862500