which until his time had been 'inexplicable'; quite unconscious, apparently, how little the term 'alternation of generations,' and the figurative expression of 'wet-nurse,' applied to the fertile larval stage of an animal, serve to acquaint us with the nature or essential conditions of the phænomena. And I may further observe, that if it were proved that the multiplication of cells by a process of continuous growth, with independent vitality,' was a process altogether distinct from that multiplication of cells in an ovum which precedes the formation of the tissues of the embryo; yet the words 'multiplication of cells, with or without continuous growth,' would equally leave us devoid of the idea of the essential force and condition of structure that make the development of a young animal from a larval polype or a wingless virgin Aphis possible. And we are interested to learn, therefore, what that may be which the reviewer calls "our philosophical interpretation of this wonderful process" (p. 203). He truly compares the fertilized ovum of the medusa-parent to the seed of the plant, the polype that grows from it with its progeny by gemmation to leaf-buds, and those discoid portions developed between the base and summit of the Strobila to flower-buds. For these discoid segments, in point of fact, when separated, become either male or female Medusa, and developing the generative organs, justify the above comparison. The reviewer says, "It is evident to us that the pile of medusadiscs is not formed by the constriction of the proper body of the original polypoid animal;" yet he afterwards states that the detachment of those discs "involves the separation of the tentacular ring from the body at the base," that it separates one end of the polypoid animal from the other, which implies something more than constriction,— constriction, viz. carried to the extent of absolute fission of the original polypoid animal. But this fission is preceded, as in the Nais and Nereis, by gemmation of the part to be separated thereby. There is doubtless a close and beautiful analogy between the stages of the development of the Medusa and those of the Tree; between the larva of the one and the leaf of the other, between the ovum of the animal and the seed of the plant. Yet this comparison does not explain the essential condition of gemmation in either: one thing unexplained cannot be made to illustrate another unknown thing. One seems to get some knowledge when it is stated that the leaf is produced by continuous growth,' and that the bud of the Hydra is produced like the leaf. And had the reviewer recognized the essential concordance between the 'continuous growth' of the conferval filament and that which augments the germinal mass in the ovum, he might have arrived at the desired explanation; but he affirms them to be altogether distinct.' Admitting the obvious analogies between the reproductive phænomena of the Medusa and the Plant, Dr. Carpenter says, "the whole of these phænomena appear to us to constitute but a single generation, instead of making two, as represented by Steenstrup ;" and, availing himself of the popular idea that a tree is one individual living thing, he turns those analogies with apparently great power against the definitions of Professor Steenstrup. It is quite true, as the able reviewer says, "that men are not in the habit of speaking of leaf-buds and the flowerbuds of a plant as of two distinct generations." But before the whole doctrine of the alternation of generations' is knocked to the ground (p. 205), those who desire to secure for the ingenious Danish Professor the credit which he really merits, may bring to his support that other and perhaps truer interpretation of the phænomena and analogies arrayed against him, to which I have before referred in regard to his opinion of the sex of the budding polype. The fertilized ovum of the medusa-parent is like the fertilized ovum of the Aphis, and the polype that grows from it resembles the first larva into which the embryo Aphis expands. From this larva are produced other wingless larvæ, which are repetitions of the producer or parent; these larvæ in the Aphis remain disconnected from each other, and do not combine to form a compound structure as in certain zoophytes; they become detached like the bulbels of the Marchantia or the lily, only they are rather more advanced in individual development. But under certain conditions a new and different series of embryos forming male and female perfect insects are produced: these too become detached, and by their inherent power of movement, through the possession of wings, they carry the germs of a new generation to a distance from the plants which their larval parents have devastated. The whole of these phænomena appear to me, as they did to Bonnet, Reaumur, Dufour, and Morren, to constitute a succession of generations,-seven, nine, or it may be eleven. But as all the larvæ are like each other, they collectively represent with the perfect insects two generations, in respect of form, and the generation of larviparous and the generation of oviparous Aphides may thus be said to alternate with each other, in accordance with Steenstrup's general expression of the fact. Now no physiologist has ever defined the series of virgin larviparous Aphides, with the ultimately produced winged males and oviparous females, as constituting a single generation, or the generation of the parts of one and the same individual; nor, if my comparison be true, have we better grounds for so defining the succession of distinct animals which have received the names of Hydra Tuba, Strobila, and Medusa, or the succession of those which have been called ciliated gemmules and planulæ, digestive polypes, generative polypes, Tintinnabula or free medusiform polypes of the Campanularia and Coryne; or the successively developed cotyledons, leaves and flowers of a plant which are combined by the stem into a seeming individual whole. The analogy between the procreating larvæ of the Aphis, the Medusa, and the Coralline is so true and so close, that if the larval Aphis be a distinct individual and not a part, so must be the strobila, the planula, and the gemmiparous leaf: if the succession of larval Aphides produced without the access of the male be truly described, as it always has been described, as a succession of generations, so must that succession of planula, polype and strobila which leads to the oviparous Medusa: and that succession of planulæ and nutritive polypes which precede the detachment of the free procreative medusoid polypes in the Coryne, and the like with the plant-generations preceding the flower. The Botanist, in fact, arrived earlier than the Zoophytologist at an intimate philosophical comprehension of the nature of his composite subjects. After Linnæus had made known with his characteristic terseness and brevity the phænomena of the transformation of the leaf into the bract, the sepal or the petal, and the retrograde change of the stamen into the petal, of the petal into the sepal, and of this again into the leaf, in his 'Philosophia Botanica*, and with more detail in the Prolepsis plantarum†'; and when the great Goethe had gathered together these and other analogous phænomena in vegetable life, and had combined them harmoniously, with true poetic insight into their essential nature, in his famous doctrine of 'Vegetable Morphology,' a reconsideration of the nature of the essentially individual plant could hardly fail to suggest itself to the thinking mind. Principium florum et foliorum idem est. Perianthum sit ex connatis foliorum rudimentis. Derivato nutrimento ad squamas amenti, destructis flosculis, mutantur in Folia. Derivato nutrimento ad flosculos amenti, fiunt folia Calyces. Macra vegetatio flores e foliis terminando producit.”—Metamorphosis + "Quando flos nascitur abeunt folia gemmacea anni sequentis in bracteas, tertii in calycem, quarti in petala, quinti in stamina, sexti in pistilla, quod a situ judicatur."-Amœnitates Academicæ, vol. vi. (1789) p. 341. The hypothesis of the succession has long been abandoned. 6 The Ur-pflanze' or ideal type-vegetable with which the 'Morphologie' of Goethe is illustrated (pl. iii. of the French translation) is an aggregate of modifications of the primitive type, not the fundamental pattern-plant itself. My idea of such, from the analogy of the typical vertebra or vertebrate skeleton*, is attempted to be expressed by the more simple forms associated together in the Frontispiece, fig. 1, and the nature of the association of the more or less modified individual plants is illustrated by the accompanying figures from the animal kingdom. The archetypal form of plant (phyton) nowhere perhaps exists in actual nature, but is presented under various modifications adapted to different conditions and functions, and deviating from the archetype as those functions become exalted. The most familiar, if not the commonest form of the individual plant, is the leaf, and it is sometimes cited as the archetypal form. In the specimens of Bryophyllum from the Physiological Series of the Museum (Nos. 2225 A & B), you may perceive the manifestation of the power of reproducing other individuals by the buds which have been developed from the angles of the marginal crenations of the leaf in No. 2225 C. the buds have developed individuals having the same form as the parent, viz. the leaf, and they have sent down an organ for independent nutriment called 'root.' I long ago pointed out in our Physiological Catalogue (vol. iv. p. 8), with regard to those very specimens, how characteristic of the leaf-individuals which enjoy unusual power of gemmiparous reproduction it was to retain much of the primitive cellular structure. The leaf, however, chiefly energises in the respiratory function, with which also a nutrient power is combined, since carbon is absorbed with the leaf is connected that slender chain of sap-vessels and air-tubes which is continued from its petiole down to the root, and forms part of the same individual with the leaf: and the aggregate of * On the Archetype and Homologies of the Vertebrate Skeleton, pl. 2, 8vo, Van Voorst, 1847. |