Mr. Darwin, however, has shown it to be highly probable that, though the injury has often partly resulted from the combination of morbid tendencies, the general cause is different. Considering the number of self-fertilized plants that were tried, he thinks it is nothing less than absurd to suppose that in all these cases the mother-plants, though not appearing in any way diseased, were weak or unhealthy in so peculiar a manner that their self-fertilized seedlings, many hundreds in number, were rendered inferior in height, weight, constitutional vigour, and fertility to their crossed offspring.[1992] Moreover, self-fertilization and close interbreeding induce sterility, and this indicates something quite different from the augmentation of morbid tendencies common to both parents.[1993] Hence it seems to be almost beyond doubt that, just as the sterility of distinct species when first crossed, and of their hybrid offspring, depends on their sexual elements having been differentiated in too great a degree, the evils of close interbreeding, or self-fertilization in plants, result chiefly from their sexual elements not having been sufficiently differentiated. But we do not know why a certain amount of differentiation is necessary or favourable for the fertilization or union of two organisms, any more than for the chemical affinity or union of two substances.[1994] It must, however, be observed that no case of complete sterility is met with in self-fertilized seedlings, as is so common with hybrids,[1995] and that interbreeding even of the nearest relations may sometimes, under very favourable circumstances, be continued through several generations without any evil results making their appearance.