This is clearly shown by hybrids never having been raised between species ranked by systematists in distinct families; and on the other hand, by very closely allied species generally uniting with facility.

But the correspondence between systematic affinity and the facility of crossing is by no means strict.
A multitude of cases could be given of very closely allied species which will not unite, or only with extreme difficulty; and on the other hand of very distinct species which unite with the utmost facility.

In the same family there may be a genus, as Dianthus, in which very many species can most readily be crossed; and another genus, as Silene, in which the most persevering efforts have failed to produce between extremely close species a single hybrid.

Even within the limits of the same genus, we meet with this same difference; for instance, the many species of Nicotiana have been more largely crossed than the species of almost any other genus; but Gartner found that N.acuminata, which is not a particularly distinct species, obstinately failed to fertilise, or to be fertilised, by no less than eight other species of Nicotiana.

Many analogous facts could be given.
No one has been able to point out what kind or what amount of difference, in any recognisable character, is sufficient to prevent two species crossing.