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judicious observers, that there can surely be no reason to distrust his accuracy in those others which have not yet been so frequently or so carefully examined."
His capital discovery was undoubtedly that of the capillary circulation of the blood, first announced in 1690, which afforded the link still wanting for the completion of the doctrine of Harvey, by showing that the blood passes from the arteries into the veins through a network of extremely minute vessels, the thin walls of which allow the fluid plasma to transude into the tissues it traverses, so as to serve for their nutrition. He first sought to discern this in the comb of a young cock, then in the ear of a white rabbit, and then in the membrane of a bat's wing; but, though in the last he was able to follow an artery to its ultimate subdivision, he found that, as soon as "it became so small as only to admit one or two globules to pass through it at a time, he then lost sight of it," partly in consequence of "the membrane of the wing being covered with a kind of scale" (epidermis). His first success was obtained with the tail of a newly-hatched tadpole, in which, he says, "I could distinctly perceive the whole circuit of the blood, in its passage to the extremities of the vessels, and in its return towards the heart," – its movement being made apparent by that of the globules carried along in its current. These corpuscles, which had been previously discovered by Malpighi, were correctly described by Leeuwenhoek as flattened circular disks in man, and as oval disks in tadpoles. He afterwards observed the capillary circulation in the tail-fins of small fishes, and recognized the ellipticity of the corpuscles in that class also. He even made out the capillary circulation in the broad thin extremities of the two smallest or hind feet of small crabs about an inch in diameter, and correctly remarked that the corpuscles of their blood were colourless and far fewer than those of fishes or tadpoles, "the globules in red blood being (I am well assured) twenty-five times more in number than those, in the same space, in the blood of a crab." To us it seems not a little surprising that his assertions in regard to the capillary circulation were deemed incredible by some of his scientific contemporaries. It is recorded, however, that Peter the Great, when passing through Delft in 1698, requested Leeuwenhoek to pay him a visit, and to bring his microscope with him, and that the czar was particularly impressed by the spectacle of the circulation in the tail of a small eel.
Among Leeuwenhoek's discoveries in the minute anatomy of man and the higher animals may be specially mentioned the tubules of teeth, the fibrous structure of the crystalline lens, the solidity of the human hair (which had been previously represented as tubular), the structure of the epidermis, and the parallel tubules of the medullary substance of the brain, – which last, however, he supposed to be vessels conveying fluid substance from the highly vascular cortical layers, for the support and nourishment of the spinal marrow and nerves. He was also an independent discoverer of the spermatozoa, although anticipated by a few months by Ludwig Hamm, a student at Leyden.
As might be expected, he made many observations on the anatomy of insects; and among the most interesting of these are his discovery of the composite structure of the eyes (which he recognized also in the eyes of the shrimp), the scales on the wings not only of moths but of the gnat, and the annular (really spiral) structure in the walls of the "vessels" (tracheæ) of their wings. He also proved that cochineal, which had been supposed to be "the fruit of some tree," is really the dried body of an insect, which he not unnaturally supposed to be allied to the ladybird. He likewise gave a very good account of the spinnerets and poison-claws of spiders, and of the comb-like appendages to their feet. He made a special study, also, of the anatomy of the flea, – besides following out its reproduction with great care, as will presently appear.
In examining the stomachs of shrimps, he found in them some minute shells, of which he figured a specimen so exactly that it can be at once recognized as a Nonionina, – probably the first recent foraminifer that had been distinctly noticed. But one of his most interesting observations is that which he made upon a small Balanus attached to a mussel-shell; for he not only gives a good figure of the animal, but describes the way in which it retreats into its shell, and closes its orifice by two shelly valves. His figure most distinctly shows its articulate character, which has only in modern times caused its removal from the molluscous to the aunulose sub-kingdom.
Not less admirable were his observations on the structure of plants. He made very careful sections of stems of the oak, elm, beech, willow, fir, and other trees, in different directions, of which he gave careful figures and descriptions, – specially noting the horizontal arrangement of the cells in the "medullary rays," and the peculiar "pitting" of the woody fibre of the fir, as well as the absence of large vessels in the latter. He also examined the structure of various germinating seeds, and gave accurate descriptions of the relation of the embryo to the cotyledons.
Although, when he adventured into physiological speculation, Leeuwenhoek's ideas (like those of the best physiologists of his time) were often very crude, his reasonings upon the facts actually observed by him are often remarkably cogent and sagacious. Thus, to estimate the insensible perspiration, he placed his hand within a dry glass jar, closed the space between its neck and his wrist by stuffing his handkerchief into it, and carefully collected and weighed the moisture which accumulated in its interior during a given time; and by a computation based on the ratio of the surface of the hand to that of the entire body he concluded that about 28 oz. of fluid are daily lost by transpiration, which is not far from the truth. So, again, he triumphantly refuted the chemical theories which then reigned in medicine, and which assumed that the blood undergoes a fermentation like that of wine or beer, by the statement that he had never seen in the blood-vessels the bubbles of gas which must be generated in them if this doctrine were correct. In one important point, however, he allowed his imagination to supplement the necessary imperfection of his observations, maintaining that each blood-disk is made up of six coherent particles, an idea probably suggested by the crenated appearance which the blood-disks often present.
It is to Leeuwenhoek that we owe the refutation of the then current biological doctrine that animals of high organization can be "produced spontaneously, or bred from corruption." This doctrine had been previously attacked by Redi, who showed that the putrefaction of meat will not engender maggots, if the access of blow-flies be prevented. But even Redi, while upholding the doctrine "Omne vivum ex vivo," believed that the insects found within the galls of plants, and the parasitic worms by which the human body is sometimes infested, are generated by a peculiar modification of the living vegetable or animal substance. It was Leeuwenhoek who first explicitly took up the position that every living organism reproduces its like, no type originating in any other way than by the ordinary generative process of its kind: "Omne vivum ex ovo." This he established by careful and prolonged observation, in a great variety of cases in which "spontaneous generation" had been reputed to take place; and he further continually adduced the great complexity of organization which his microscopic researches had revealed in what had been previously regarded as creatures of the lowest grade, as an argument against the doctrine that they are "bred from corruption." Thus he followed out the whole history of oak-galls, and showed that they are a product of a peculiar vegetable growth, excited by the insertion of an egg by the winged insect, and supplying the maggot, when hatched, with food. So, again, he showed that the weevils of granaries, then commonly supposed to be bred from wheat, as well as in it, are nothing else than grubs hatched from eggs deposited by winged insects; and he practically applied this conclusion, by recommending that granaries thus infested should be repeatedly fumigated with sulphur at the time when these insects come forth, so as to kill them before they deposit eggs. His chapter on the flea, in which he not only describes its structure, but traces out the whole history of its metamorphoses from its first emergence from the egg, is full of interest, – not so much for the exactness of his observations, as for its incidental revelation of the extraordinary ignorance then prevalent in regard to the
