The most significant fact in this connection is that the more widely different two species are, so that they are placed in different families, so much the less probable is it that cross-fertilization will produce any result. From this condition of infertility there may be traced a gradation between less different forms of the same genus to almost complete, or even complete, fertility between closely similar species. Darwin further points out that: “The hybrids raised from two species which are very difficult to cross, and which rarely produce any offspring, are generally very sterile; but the parallelism between the difficulty of making a first cross, and the sterility of the hybrids thus produced—two classes of facts which are generally confounded together—is by no means strict. There are many cases, in which two pure species, as in the genus Verbascum, can be united with unusual facility, and produce numerous hybrid offspring, yet these hybrids are remarkably sterile. On the other hand, there are species which can be crossed very rarely, or with extreme difficulty, but the hybrids, when at last produced, are very fertile. Even within the limits of the same genus, for instance in Dianthus, these two opposite cases occur.” In regard to reciprocal crosses Darwin makes the following important statements: “The diversity of the result in reciprocal crosses between the same two species was long ago observed by Kölreuter. To give an instance: _Mirabilis jalapa_ can easily be fertilized by the pollen of _M. longiflora_, and the hybrids thus produced are sufficiently fertile; but Kölreuter tried more than two hundred times, during eight following years, to fertilize reciprocally _M. longiflora_ with the pollen of _M. jalapa_, and utterly failed.”