The Budding-out of New Animals (Hydra): another Form of Propagation without Mating (Aphids).
In the year 1744 a young Genevese, Abraham Trembley, tutor in the family of Bentinck, who was then English resident at the Hague, rose into sudden fame by a solid and well-timed contribution to natural history. Trembley and his pupils used to fish for aquatic insects in the ponds belonging to the residence, and in the summer of 1740 he happened to collect some water-weeds, which he put into a glass vessel and set in a window. When the floating objects had come to rest, a small green stalk, barely visible to the naked eye, was found attached to one of the plants. From one end of the stalk filaments or tentacles were seen to project, and these moved slowly about. When the vessel was shaken the stalk and tentacles contracted, but soon extended themselves again. Was this object a plant or an animal? Its shape and colour were those of a plant, and sensitive plants were known which drooped when touched or shaken. Further observation showed that it could move from place to place, which favoured the animal interpretation. Trembley determined to cut the stalk in two; if the halves lived when separated the fact would favour the plant-theory. The halves at first gave no signs of life beyond occasional contraction and expansion, but after eight days small prominences were seen on the cut end of the basal half. Next day the prominences had lengthened; on the eleventh day they seemed to be growing into tentacles. Before long eight fully formed tentacles were visible, and Trembley had two complete specimens in place of one; both were able to move about.
After four years of observation a handsome quarto volume was published, which told the history of "The freshwater Polyp," a name suggested by Réaumur; the Latin name of Hydra was given by Linnæus. Hydra had been discovered and slightly described forty years before by Leeuwenhoek, who had seen two young polyps branching from one parent and spontaneously becoming free. Trembley made out all that a simple lens, guided by a skilful hand and a keen eye, could discover. Thirteen plates were admirably engraved by another amateur, Pierre Lyonet, who was in all respects a fit companion for Trembley. It was proved that Hydra preyed upon living animals, especially upon the Daphnia or water-flea. When it was well nourished it branched spontaneously again and again, forming a compound mass made up of scores or even hundreds of polyps, all connected with a single base. The power of locomotion and the power of devouring prey were held to settle the animal nature of Hydra, a decision to which zoologists have ever since adhered. Lyonet went on to try the effect of division upon some common freshwater worms, and found that each part grew into a complete worm. Artificial division is not indispensable; in the worm called Nais division takes place spontaneously at certain seasons, one segment dividing repeatedly, so as to form the segments of a complete new individual. The process may be repeated until a chain of worms is produced, which at length breaks up.[14]
A nail was thus driven in a sure place. The conception of an animal was enlarged, for it was shown that an animal may branch and multiply in a way hitherto supposed to be peculiar to plants. The old connecting links between animals and plants (zoophytes, sponges, etc.) had never been really investigated; no one knew what sort of organisms formed or inhabited their plant-like skeletons. But Hydra, thanks to Trembley's description, furnished a clear example of an animal which possessed some of the attributes of a plant. Forms more ambiguous than Hydra, such as Volvox and Euglæna, were ultimately to make the distinction between animal and plant very uncertain and shadowy. It was Hydra that gave the first clue to the structure of the zoophytes, and dispelled the false notion that corals are plants, bearing flowers, fruits, and seeds.
Baer[15] has remarked that Trembley's discovery appreciably modified the teaching of physiology by showing that an animal without head, nerves, sense-organs, muscles, or blood may perceive, feed, grow, and move about.
At the time when Trembley was demonstrating the asexual propagation of Hydra, Bonnet (supra, p. 45) was demonstrating the asexual propagation of aphids. Both naturalists were natives of Geneva, and both, as well as their associate Lyonet, were in a sense pupils of Réaumur, who not only set them an admirable example, but directed their attention to promising researches and discussed with them the conclusions which might be drawn. Réaumur's experience had seemed to confirm Leeuwenhoek's statement (supra, p. 34) that aphids produce young alive, even though no males are to be found among them; but unlucky accidents defeated his intention to confirm it by experiment, and when Bonnet asked him to suggest a piece of work Réaumur gave him the aphid problem.[16]
Bonnet filled a flower-pot with moist earth, introduced a food-plant together with a single new-born aphid, and covered all up with a bell-jar. In twelve days the aphid produced its first young one; in a month ninety-five had been born from the same unfertilised parent. As many as five generations were obtained without the intervention of a male, each successive parent having been isolated from the moment of its birth. It was, however, discovered, apparently by Lyonet, that though viviparous reproduction without males went on regularly so long as food was plentiful, males appeared towards the end of summer, and fertilised the eggs which were destined to outlast the winter.
The aphids added a new and peculiar example to the known cases of asexual propagation (plants and Hydra). Much discussion followed, but the physiology of that age (and the same is true of the physiology of our own age) was unable to reveal the full significance of the observed facts. Insects have since furnished many instances of unfertilised eggs which yield offspring. One such instance was already recorded, though neither Leeuwenhoek, Réaumur, nor Bonnet knew of it. In the year 1701 Albrecht of Hildesheim placed a pupa in a glass vessel and forgot it. A moth hatched out and laid eggs, from which a number of caterpillars issued.
Lyonet, whom we have more than once had occasion to mention, afterwards became celebrated as the author of one of the most laborious and beautiful of insect-monographs. The structure of the larva of the goat-moth was depicted by him in eighteen quarto plates, crowded with detail.