In parthenogenesis no amphimixis occurs, but neither does any reduction of the number of the ids to one-half; all the ids present at the beginning of parthenogenesis are retained; they are only no longer mingled with strange ids. In inbreeding both amphimixis and reduction take place, but the former soon ceases to convey any really strange ids to the germ-plasm, but only the same as those which it already contains, so that a rapidly increasing monotony of the germ-plasm must result. To this must be added the possibility that among the few ids which now--many times repeated--form the germ-plasm, some must occur which exhibit unfavourable variational tendencies in one or many determinants, and then the same thing will occur which usually occurs in experimental inbreeding of domesticated animals, namely, _degeneration of the progeny_. In parthenogenesis the case is otherwise; unfavourable variational tendencies, as soon as they attain selection-value, are, so to speak, eliminated root and branch, because the individuals which exhibit them, and their whole lineage, are exterminated, without their having any effect upon the other collateral lines of descent. A purely parthenogenetic species will, therefore, not degenerate as long as individuals of normal constitution are present, for these reproduce with perfect purity. But if in later generations unfavourable variational tendencies crop up in the germ-plasm through germinal selection, the process of personal selection will be reinforced on these or on their descendants, and it is conceivable, and even probable, that in perfectly adapted species parthenogenesis may last for a very long time without doing any injury to the constitution of the species. The same is true of purely asexual reproduction, to the investigation of which we shall now turn. Let us leave out of account the simplest animals (Monera) without amphimixis, which we have already discussed. In simple animals reproduction by budding or by fission is frequent, or it occurs in alternation with sexual reproduction; in higher animals, Arthropods, Mollusca, and Vertebrates, asexual reproduction is wholly absent. In plants it plays an enormously greater part, and what is called 'vegetative reproduction,' which is purely asexual without any amphimixis, is to be found in all groups of plants, especially in the form of budding and spore-formation, besides which there is multiplication by runners, rhizomes, tubers, bulbs, and bulbils. In most cases there is, in addition to the purely asexual reproduction, so-called sexual reproduction associated with amphimixis, and often the sexual and asexual generations alternate with each other, so that 'alternation of generations' occurs, as is common in lower animals, especially polyps, medusæ, and worms. But it sometimes happens among plants that the sexual reproduction is absent, and that a species reproduces by the asexual mode only, and this is the case which we must now consider more closely.