* Owen, Parthenogenesis, p. 24.

1 "The multiplication of these little creatures is infinite and almost incredible. Providence has endued them with privileges promoting fecundity which no other insects possess: at one time of the year they are oviparous, at another, viviparous; and, what is most remarkable and unparalleled, the sexual intercourse of one original pair serves for all the generations which proceed from the female for a whole succeeding year. Reaumur has proved that in five generations one Aphis may be the progenitor of 5,904,900,000 descendants; and it is supposed that in one year there may be twenty generations." - Kirby and Spence, Introd. to Entom. vol. i. p. 175.

(896). To facilitate the comparison thus instituted by Steenstrup, Professor Owen has devised the diagrams copied in the opposite figure, which place before the eye the whole of this most interesting subject*.

(897). The pollen-tube or filament (fig. 172, 1, a) discharged from the pollen-cell (a') in the plant, represents the spermatozoon (a 2 & 3) in the animal; its contents (whether by endosmose or perforation is immaterial) are received by the ovule (b l), which is afterwards discharged and becomes free. Under favourable circumstances the formation of the embryo takes place with manifold modifications, but essentially by the multiplication of cells, according to a process which is as much entitled to be called continuous growth as that process in the formation of the Conferva. The embryo (c) proceeds to develope the radicle and the plumula (d) by the metamorphosis and coalescence of certain of the impregnated cells, retaining the major part, however, as cells; and thus the first individual plant or pair of individuals, as in Dicotyledons, is established.

(898). The ovum (fig. 172, 2, b) of the zoophyte proceeds to develope its free locomotive embryo (c) by an analogous multiplication of cells, certain of which are metamorphosed into an external skin with vibratile cilia; the embryo settles, subsides, shoots out rays analogous to the radicle of the plant, but for attachment only, and grows afterwards as a stem, from which a polyp (d) is speedily developed, answering to the first cotyledonal leaf or leaves in the plant (fig. 172,1, d.) Both plant and zoophyte proceed to develope by gemmation, the one a succession of leaves (e e), the other of polyps (e e) associated by the continuous growth of the connecting parts; and finally the plant, by a metamorphosis of part of the stem and certain leaves, produces the flower or fructification (f, g, h, i); and the zoophyte, by a modification also of its stem and certain polyps, produces an "ovarian vesicle" (f), or a modified polyp (g), or a medusiform individual (l), which is set free: in both cases the end to be attained is the diffusion of the species by means of impregnated seeds or ova.

* Vide Owen, Parthenogenesis, p. 58 et sea.

(899). Now if we compare fig. 172, 3, in which are represented the corresponding stages intervening between the ovum and the perfect male and female individuals of the Aphis, with fig. 172, 1 & 2, the analogy between these stages in the plant, the polyp, and the insect will be seen to be both true and close. The spermatozoon (a) of the male Aphis (h) answers to the pollen-filament (fig. 172, 1, a) of the male leaf or stamen (h.) The ovum (b) of the female Aphis (i) answers to the ovule (b) of the female leaf or pistil (i); by their combination the impregnated ovum results. The same processes of cell-formation ensue, and the embryo Aphis (d) is formed by the combination and metamorphoses of certain of these secondary germ-cells; but it retains the rest as a germ-mass in its interior, which may be compared with the cells of the pith in the plant, and with the cells or nuclear granules in the corresponding more fluid part of the pith of the polyp. Under favourable circumstances of nutriment and warmth, certain portions of the retained germ-mass repeat the process of embryonic formation, and a larval individual (fig. 172, 3, e) like that from the ovum is thus produced, which is only not retained in connexion with its parent because the abdominal integument is not co-extended with it.

Comparative view of the reproductive process in a Plant, a Campanularian zoophyte.

Fig. 172. Comparative view of the reproductive process in a Plant, a Campanularian zoophyte, and a female Aphis, the corresponding parts being indicated by similar letters, as explained in the text.

(900). The generation of a larval Aphis may be repeated from seven to eleven times without any more accession to the primary spermatic virtue of the retained germ-masses than in the case of the zoophyte or plant: one might call the generation an "internal gemmation;" but this phrase would not explain the conditions essential to the process, unless we previously knew those conditions in regard to ordinary or external gemmation. At length, however, the last apterous or larval Aphis so developed proceeds to be metamorphosed, as it is termed, into a winged individual, in which, only, the fertilizing filaments are formed, as in the case of the stamens of the plant (h); another larval Aphis (fig. 172, 3, i) perfects the female generative organs, and developes the ovules, as in the case of the pistil (fig. 172, 1, i.) We have, in fact, at length male (h) and female (i) individuals, preceded by reproductive individuals (e e) of a lower or arrested grade of organization, analogous to the gemmiparous polyps of the zoophytes (fig. 172, 2, e e) and the leaves (fig. 172, 1, e e) of the plant.

(901). The process of development in the Aphides is, for its better intelligibility, described above by Professor Owen as one of a simple succession of single individuals, but it is much more marvellous in nature. The first-formed larva of early spring procreates not one but eight larvae like itself in successive broods; and each of these larvae repeats the process; and it may be again repeated in the same geometrical ratio, until a number which figures only can indicate, and which language almost fails to express, is the result. The Aphides, generated from virgin parents by this process of internal gemmation, are as countless as the leaves of a tree to which they are so closely analogous. The wingless larval Aphides are not very locomotive; they might have been attached to one another by continuity of integument, and each have been fixed to suck the juices from the part of the plant where it was brought forth. The stem of the rose might have been incrusted with a chain of such connected larvae, as we see the stem of a fucus incrusted with a chain of connecting polyps, and only the last developed winged males and oviparous females might have been set free.