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.

It is also a remarkable fact that hybrids raised from reciprocal crosses, though of course compounded of the very same two species, the one species having first been used as the father and then as the mother, though they rarely differ in external characters, yet generally differ in fertility in a small, and occasionally in a high degree.