We shall observe in the third place, that if we compare the figures a, b, c, d, ([PLATE III.]) copied from the Philosophical Transactions, with those which Leeuwenhoek had engraved many years after, ([PLATE IV.]) we shall find considerable difference, especially in the figures of the dead animals, of a rabbit and in those of a dog, (which plate we have also copied for the satisfaction of our readers) from all which we may conclude, that Leeuwenhoek has not always observed objects entirely alive: that the moving bodies, which he looked upon as animals, appeared to him under different forms; and that he has varied in his assertions, with a view of making the species of men and animals perfectly consistent; he has not only varied in the basis of these experiments, but even in the manner of making them, for he expressly says, that he always diluted the liquor with water, in order to separate, and to give more motion to these animalcules: nevertheless, in his letter to Lord Brouncker, he says, that having mingled an equal quantity of rain water with the seminal liquor of a dog, in which he had before perceived an infinity of living animalcules, yet the mixing of this water killed them. The first experiment of Leeuwenhoek's therefore was made, like mine, without any mixture; and it even seems, that he was not of opinion to mix any water with the liquor till a long time after; because he thought he had discovered, by his first essay, that water caused the death of the animalculæ; which however is not the fact. I think that the mixture of the water only dissolves the filaments very suddenly; for I have seen but very few filaments in all the experiments I have made after mixing the water with the seminal liquor.
As soon as Leeuwenhoek was persuaded that spermatic animals were transformed into men, and other animals, he imagined he saw two sorts in the seminal liquor of every animal, the one male, and the other female; and this difference, according to him, served not only for the generation of themselves, but for the production of males and females, which was very difficult to conceive by a simple transformation. He speaks of the male and female animalcule, in his letter printed in the Philosophical Transactions, No. 145, and in many parts of his works,[U] but he does not describe the difference of these male and female animalcules, and which in fact never existed but in his own imagination.
[U] See vol I. page 163, and vol. III. page 101, of his works.
The famous Boerhaave having asked Leeuwenhoek, if he had not observed in spermatic animals different degrees of growth and size? Leeuwenhoek answered, that having dissected a rabbit, he observed in the semen an infinite number of living animals. "Incredibilem, says he, viventium animalculorum, numerum conspexerunt, cum hæc animalcula scypho imposita vitreo & illic emortua, in rariores ordines disparassent, & per continuos aliquot dies sæpius visu examinassem, quædam ad justam magnitudinem nondum excrevisse adverti. Ad hæc quasdam observavi particulas perexiles & oblongas, alias aliis majores, &, quantum oculis apparebat, cauda destitutas; quas quidem particulas non nisi animalcula esse credidi, quæ ad justam magnitudinem non excrevissent."[V] Here then are animalcules of different sizes, some with tails and others without, which much better agrees with my experiments, than with Leeuwenhoek's own system. We differ only in one particular; he says, that those without tails were young animalculæ, which were not arrived at their full growth; while I, on the contrary, have seen these pretended animals quit the filaments with tails or threads, and afterwards lose them by degrees.
[V] See vol. IV. pages 280 and 281.
In the same letter to Boerhaave, he says, in the semen of a ram, he perceived animalcules following each other in swarms like a flock of sheep. "A tribus circiter annis testes arietis, adhuc calentes, ad ædes meas deferri curaveram, cum igitur materiam ex epididymibus eductam, ope microscopii contemplarer, non sine ingenti voluptate advertebam animalcula omnia, quotquot innatabant semini masculino, eundem natando cursum tenere, ita nimirum ut quo itinere priora prinatarent eodem posteriora subsequerentur, adeo ut hisce animalculis quasis sit ingenitum, quod oves factitare vidimus, scilicet ut precedentium vestigiis grex universus incedat." This observation, which Leeuwenhoek made in 1713, and which he looks upon as singular and novel, proves to me, that he had never examined the seminal liquors of animals with attention, at least sufficient to give very exact descriptions of them. Leeuwenhoek was sixty-one years old in 1713, had made microscopical observations for more than forty-five years, had published the discovery of spermatic animals for about thirty-six years, and then, for the first time, saw in the seminal liquor of a ram, what is seen in all seminal liquors, and what I have described in Experiment [IX] in the seed of a man; Experiment [XII] in the seed of a dog; and in Experiment [XXIX] in that of a bitch. It is not necessary to suppose the spermatic animals of the ram are endowed with instinct, to explain the floating of these animals, in flocks like sheep, since those of a man, dog, or bitch, does the same; and which motion depends solely on particular circumstances, whose principle is, that all the fluid matter of the seed is on one side, while the thick matter is on the other; for then all the bodies in motion will be disengaged from the mucilage, and follow the same road into the most fluid part of the liquor.
In another letter, written the same year, to Boerhaave, he relates some further observations he made on rams, and says, that he has seen, in the vasa deferentia, flocks of animals which float all on one side, and others which go in a contrary direction; and he adds, "Neque illud in unica epididymum parte, sed & in aliis quas præcideram partibus, observavi. Ad hæc, in quadam parastatarum resecta portione complura vidi animalcula, quæ necdum in justam magnitudinem adoleverant, nam et corpuscula illis exiliora & caudæ triplo breviores erant quam adultis. Ad hæc, caudas non habebant desinentes in mucronem, quales tamen adultis esse passim comperio. Præterea in quandam parastatarum portionem incidi, animalculis quantum discernere potui, destitutam, tantum illi quædam perexiguæ inerant particulæ, partim longiores, partim breviores, sed altera sui extremitate crassiunculæ; istas particulas in animalcula transituras esse non dubitabam." It is easy to see, by this passage, that Leeuwenhoek had seen, in this seminal liquor, what I found in all; that is to say, moving bodies of different sizes, figures, and motions; and which agrees much better with the idea of organic particles in motion than of that with real animals.
It appears, therefore, that Leeuwenhoek's observations are not contrary to mine, although he has drawn very different conclusions from them. I am persuaded that if any person would take the trouble of making the like experiments they would not have any difficulty in discovering from whence these differences proceed, and would find that I have advanced nothing which is not conformable to truth; and to enable the reader to decide thereon, I shall subjoin a few remarks.
The filaments I have spoken of are not always to be perceived in the seminal liquor of a man. To discover them it must be examined the moment it is taken from the body, and even then it will sometimes happen that there is not one to be seen. Sometimes the seminal liquor presents, especially when it is very thick, only large globules, which may be even distinguished with a common lens. By inspecting them with the microscope they appear like young oranges; they are very opaque, and a single one often fills up the whole table of the microscope. The first time I saw these globules I thought they were some foreign matters fallen into the liquor, but having examined different drops I discovered that the whole was composed of these thick globules. I selected one of the roundest, and whose size was such that, its centre being in the middle of the table of the microscope, I could at the same time observe the whole circumference; at first it was absolutely opaque; a short time afterwards I perceived a bright luminous ring to form on its surface, which remained about half an hour, and then approached by degrees towards the centre, which became clear, and of different colours, while the remainder of the globule continued opaque. This light, which brightened in the centre of the globule, resembled those seen in the great air bubbles. The globule then began to get a little flat, and acquire a small degree of transparency. Having examined it more than three hours I perceived no more alteration, nor any appearance of motion, either internally or externally. I then imagined, that by mixing this liquor with water, these globules might be changed; in fact they did change, but they presented only a transparent and homogeneous liquor, wherein was nothing remarkable. I suffered the seminal liquor to liquefy of itself, and examined it at the end of six, twelve, and twenty-four hours, but saw nothing more than a fluid; without the smallest resemblance of life or motion. I only relate this observation to shew that there are times when the common phenomena are not to be seen in the seminal liquor.