Only the higher Vertebrates breathe by means of lungs; fishes and numerous aquatic Invertebrates breathe by gills, and insects by air-tubes (tracheæ). [Illustration: FIG. 7.—The Small-winged Gall-fly, _d_ (_Andricus terminalis_), lays its eggs separately in the rootlets of oak. Root-galls (_a_) result from this, and inside of each of them a larva develops which, after a metamorphosis, becomes a relatively large, wingless gall-fly (_c_) known as _Biorhiza aptera_. This pierces the oak-buds in early spring, and lays a large number of eggs in them; from part of the bud is formed a large juicy gall (_b_), containing several larvæ, from which the small-winged gall flies (_d_) develop. The species here represented exist, therefore, in two forms, _e_ and _d_ (Heterogeny). ] While _Nutrition_ is the life-process which shields the _individual_ from death, Reproduction serves to maintain the _species_. It is familiarly known that the offspring generally resemble their parents. But it is also a fact recognized by the stock-breeder, that a particular animal will not only transmit several of its _own_ characteristics to its offspring, but perhaps also various characteristics of the grandparents or of animals belonging to still more remote generations, although these characteristics are not visible in the animal which is actually breeding (_Reversion_, _Atavism_). Among insects and the lower animals there are species which, as adult animals, appear not in _one_ form, but two or several. In this case, as a regular thing, the offspring does not resemble the parents, but the grandparents, great-grandparents, or a still earlier generation. The older observers have placed the offspring and the parents, and sometimes the grandparents too, of the same animal species in different species, or even genera or families, until newer researches on the reproduction and development of these animals have proved them to belong to one and the same species (see Fig. 7 and explanation). The method of reproduction by which a species appears in two or several forms is distinguished as _heterogeny_ and _metagenesis_, or _alternation of generations_. In the first (Fig. 7) sexually reproducing animals alternate with other sexual animals. It may be that these are of separate sexes, or else they may possess both male and female organs (_hermaphrodite_). In metagenesis a sexual generation regularly alternates with one or several generations reproducing asexually. The animal kingdom falls (cf. p. 2) into sub-kingdoms or main divisions. Seven of these are commonly distinguished: I. Backboned animals; II. Jointed-limbed animals; III. Worms; IV. Molluscs; V. Echinoderms; VI. Cœlenterates; VII. Protozoa. First Sub-Kingdom: VERTEBRATA (BACKBONED ANIMALS).