Instances sometimes occur of Species, or Varieties, gaining partial possession of the bodies of one another[…]Such instances, however, are very rare, and probably never take place between Varieties belong to the same Primary Species: for although it is an occurrence that evinces an intimate affinity between the plant that confers its distinctions, and that which is the accipient, yet the existence of the conferent in an entire state, would induce us to conclude that there exists a disparity between them which we cannot reasonably suppose to exist between Varieties that have sprung from the same Species.
Source: wiktionary