The diversity of the result in reciprocal crosses between the same two species was long ago observed by Kolreuter.

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.