Previous to the successful experiments of this gentleman, Leibnitz and Boerhaave, as well as some of the ancient philosophers, reflecting on the various gradations in the scale of animated nature, had endeavoured to prove that there might be degrees of life between the animal and the plant, and that animals might be found which would propagate by slips, like plants. These conjectures were verified by Trembley, but not in consequence of any pre-conceived ideas in favour of such a supposition; on the contrary, it was only by repeated observations that he could destroy his own prejudices, and join these wonderful beings to the animal kingdom.[106]
[106] A great part of the knowledge of the ancients consisted in an extensive variety of ingenious hypotheses, the result of intense study and application; and it need not excite surprize, if, amongst a number of suppositions, some of them have since been found conformable to truth.
The moderns, animated by the example of the great Bacon, by an abundance of experiments frequently repeated, and the assistance of good instruments, have introduced unquestionable demonstration in the place of speculation; this renders the present philosophy very far superior to that of the ancients.
Thus it is with respect to the subject now under consideration; many of the ancients conjectured that animated beings might exist possessed of the wonderful properties of the hydra; that some of them, however, were even witnesses of the fact, cannot well be disputed; though it may be fairly presumed, that their knowledge of this animal, comparatively with that we are now in possession of, was very circumscribed and imperfect.
St. Augustine relates that one of his friends performed the experiment before him, of cutting a polype in two, and that immediately the two parts thus separated betook themselves to flight, moving with precipitation different ways. The original passage is too long to be here inserted, but may be found in his work “De Quantitate Animæ,” c. 62, p. 431, col. 1.
Aristotle, speaking of insects with many feet, expresses himself nearly in the same manner; without naming the particular creatures to which he alludes, he observes, that there are of these animals or insects, as well as of plants and trees, some that propagate themselves by shoots; and, as what were but the parts of a tree before, become thus distinct and separate trees; so, in cutting one of these animals, the pieces which before composed altogether but one animal, become suddenly so many distinct individuals. And he adds, that the soul in these insects is in effect but one, though multiplied in its powers, as it is in plants. Aristot. de Histor. Animal. tom. 1, lib. 4, cap. 7, pag. 824, & de Part. Animal. lib. 4, tom. 1, cap. 6, pag. 1029. &c. This will suffice to shew that the ancients were not entirely unacquainted with the subject before us; though it does not derogate from the merit of Leeuwenhoeck, Trembley, and other ingenious naturalists, by whose assiduous and patient investigations we have obtained a more perfect knowledge of this astonishing class of animated beings. Edit.
Though natural history is so fruitful in extraordinary facts, it has hitherto produced none so singular as the various properties of the different species of the hydra.
I shall endeavour, first, to trace the progress of this discovery, in which we shall see with what sage caution and accuracy Trembley, and other naturalists examined this wonderful phænomenon, and what accumulated evidence was judged necessary to establish the fact.
We find M. Trembley writing in January, 1741, to M. Bonnet, that he did not know whether he should call the object which then engaged his attention, a plant or an animal. “I have studied it,” says he, “ever since June last, and have found in it striking characteristics of both plant and animal. It is a little aquatic being. At first sight, every one imagines it to be a plant; but if it be a plant, it is sensitive and ambulant; if it be an animal, it may be propagated by slips or cuttings, like many plants.” It was not till the month of March, in the same year, that he could satisfy himself as to their nature.
When Reaumur saw, for the first time, two polypes formed from one that he had divided into two parts, he could hardly believe his eyes; and even after having repeated the operation an hundred times, and again examined it an hundred more, he says that the sight was not become familiar to him.