To give an instance: Mirabilis jalapa can easily be fertilised by the pollen of M.longiflora, and the hybrids thus produced are sufficiently fertile; but Kolreuter tried more than two hundred times, during eight following years, to fertilise reciprocally M.longiflora with the pollen of M.
jalapa, and utterly failed.

Several other equally striking cases could be given.

Thuret has observed the same fact with certain sea-weeds or Fuci.

Gartner, moreover, found that this difference of facility in making reciprocal crosses is extremely common in a lesser degree.

He has observed it even between closely related forms (as Matthiola annua and glabra) which many botanists rank only as varieties.