Such cases are highly important, for they prove that the capacity in any two species to cross is often completely independent of their systematic affinity, that is of any difference in their structure or constitution, excepting in their reproductive systems.

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.