In the first place, he points out that all species are not infertile when crossed with other species. The sterility of various species, when crossed, is so different in degree, and graduates away so insensibly, and the fertility of pure species is so easily affected by various circumstances, that it is most difficult to say where perfect fertility ends and sterility begins. “It can thus be shown that neither sterility nor fertility afford any certain distinction between species and varieties.” Darwin cites several cases in plants in which crosses between species have been successfully accomplished. The following remarkable results are also recorded: “Individual plants in certain species of Lobelia, Verbascum, and Passiflora can easily be fertilized by pollen from a distinct species, but not by pollen from the same plant, though this pollen can be proved to be perfectly sound by fertilizing other plants or species. In the genus Hippeastrum, in Corydalis as shown by Professor Hildebrand, in various orchids as shown by Mr. Scott and Fritz Müller, all the individuals are in this peculiar condition. So that with some species, certain abnormal individuals, and in other species all the individuals, can actually be hybridized much more readily than they can be fertilized by pollen from the same individual plant!”[14] Footnote 14: A somewhat parallel case has recently been discovered by Castle for the hermaphroditic ascidian _Ciona intestinalis_. In this case the spermatozoa of any individual fail to fertilize the eggs of the same individual, although they will fertilize the eggs of any other individual. In regard to animals, Darwin concludes that “if the genera of animals are as distinct from each other as are the genera of plants, then we may infer that animals more widely distinct in the scale of nature can be crossed more easily than in the case of plants; but the hybrids themselves are, I think, more sterile.”