Amazing Animalcules, Analogies and the Limits of Imagination

In the early modern era there were really two kinds of microscopic investigations. One, which was the most common and most loved by the Royal Society of England, was the first and best known. This was the microscopy of Robert Hooke and most of the other microscopists of his day, which sought to elaborate diminutive structures in nature to demonstrate the power of instrumentation, the omnipotence of God and the value of the mechanical philosophy. On the other hand there was another kind of microscopy much less common, but which had its adherents sparsely scattered throughout Europe. This was the microscopy focused on things which today we would call truly microscopic. When reviewing the nature of this divide, it could be said that the microscopy of the truly microscopic was constantly beset with technical and philosophical problems for those engaged in it. Unlike the more widespread anatomical work of Hooke’s microscopy, these observations spoke more of God’s inscrutability than His omnipotence, the instrument’s failings rather than their usefulness and the limits of mechanical philosophy. The prime example of this divide would be the man many consider the father of microbiology, Antony van Leeuwenhoek, who in his own person, exploits and mistakes represents better than any of his peers these most elucidating difficulties of the seventeenth and eighteenth centuries.

If these two kinds of microscopy did exist, and if the first was clearly more common than the other, there should be a telling trail of topics which could show this trend. If one takes a look through Robert Hooke’s Micrographia one sees a large number of insects, some manmade objects and some flora, the vast majority of which are accompanied by a side illustration of how the object would appear to look to the unaided eye. Indeed, the microscope first became an object of interest for its ability to show with startling size the minute parts of insects and one of the first microscopic illustrations was made in 1630 of a Bee’s anatomy. Going through the literature of the time it is also apparent that the vast majority of microscopic work was being directed to the minute parts of diminutive bodies. In the articles on microscopy present in the Philosophical Transactions, approximately one hundred and twenty articles are about diminutive structures, whereas only thirteen are clearly stated as being reserved for what we today would call the truly microscopic. The fact that a vast number of these articles were written by Leeuwenhoek himself further demonstrates how much the study of diminutive structures was favoured over subjects truly microscopic, even by one of their most famous investigators.

The first obstacle to true microscopic research was the difficulty in making instruments powerful enough to even see that there was a realm of tiny things outside of unaided human perception. Indeed, for the most part early modern microscopists did not have a solid reason to believe that there could be things too small for the human eye to see, if only unclearly. There was even the view held by St. Aquinas that it was not possible: “that there should be certain parts of flesh and bone which are non-sensible because of smallness”. Yet such things had been postulated by a few and were theoretically accepted before being actually observed in nature. Nicholas Malebranche, while contemplating the nuances and power of God, commented that there was in fact nothing inherently impossible about there being an infinite regression of ever smaller animals.

The technical difficulty was overcome by Leeuwenhoek with his simple microscope, making the discovery of microorganisms possible. His microscopes tended to be of a relatively straightforward composition when compared to those produced by his contemporaries, so much so that Marian Fournier in her work The Fabric of Life: Microscopy in the Seventeenth Century, describes them as being “of a rather wayward design”. Yet from studying Leeuwenhoek’s still existent tools, these simple microscopes could magnify from between 3 and 266 times, and some argue that he made others capable of magnifying up to 395 times. One of the secrets of Leeuwenhoek’s microscopes was the smallness of their lenses, which gave them a greater resolution than the larger ones used in compound microscopes. Yet Leeuwenhoek never disclosed his method of making lenses, and their construction and maximum capacities remains a mystery to this day. This fact was much to the chagrin of Hooke, who would later be forced to design his own simple microscope to corroborate Leeuwenhoek’s claim about the existence of microorganism. Contemporary studies seem to indicate that he may have even had some method for obtaining dark-field illumination to bring out the contrast between his observed specimen and their surroundings, which would have even more greatly enhanced his observations.

Leeuwenhoek’s secrecy about his methods meant that the existence of microorganisms from the beginning was met with scepticism. As Edward G. Ruestow puts it in The Microscope in the Dutch Republic: The Shaping of Discovery: “Observations that depended on such exceptional abilities, techniques, and instruments – particularly when partially shrouded in secrecy – were all the more difficult for others to reproduce”. After describing his discovery of microscopic “animalcules” to the Royal Society of London, Nehemiah Grew was asked to reproduce Leeuwenhoek’s findings. Grew was unable to do so, and it was only after Leeuwenhoek had forwarded the testimonials of eight respectable visitors as proof of his discoveries that Hooke manage to corroborate his findings. Despite the proof of this startling new discovery “none of the members [of the Royal Society] began original investigations at that period”, into these creatures save for Leeuwenhoek himself.

Robert Hooke more often proclaimed that it was God’s power that was most evidently seen in a study of the diminutive world, stating in his Micrographia that among “natural forms there are some so small […] that the more we magnify the object […] the more we discover the imperfections of our senses, and the Omnipotency and Infinite perfections of the great Creatour”. Yet Leeuwenhoek, upon viewing the microscopic world, claimed the things that he saw were almost unfathomable and ineffable to the human mind, for: “he spoke now as well of the secrets […] in all things, secrets the human mind by itself could often never conceive”. A noticeable divide between those few who studied the truly microscopic and those who studied the diminutive is the different impressions these two microscopic disciplines imposed upon the viewers religious sensibilities. For the vast majority of the Royal Society there was a prayer-like quality to their natural investigations yet: “the identification of science with worship was notably absent from Leeuwenhoek’s explanations of why he pursued his researches”.

Despite this there was a religious component to Leeuwenhoek’s microscopic investigations which was similar to those of other microscopists of the time, namely, the wonder in God’s creations. Yet where others saw a display of power that could be known further, Leeuwenhoek saw in his studies of microorganisms only an ever deepening curtain of mystery, despite his continual efforts at elucidation. Particularly after 1690: “Leeuwenhoek’s letters repeatedly emphasized the incomprehensible smallness and enduring concealment of nature’s parts. During these later years, the theme of ‘how little we know’ also became commonplace in his letters”.

It was in this concealment that Leeuwenhoek saw the depths of God’s mysteries, and it was also here where today he is considered to have made the most blatant of errors. Leeuwenhoek believed that the smallest of animals had the same “perfection” as larger ones. What this entailed, however, was the belief that microscopic organisms possessed muscles, nerves, hearts and indeed all the organs of macroscopic life. These organs were necessary, thought Leeuwenhoek, for the very existence of life, and the fact that his best instruments were unable to make out these minute details only reinforced for him God’s method of working in the world through mysteries. Two examples of this tendency can be seen in Leeuwenhoek’s insistence that there were as-yet unseen “legs” on a number of the more inscrutable microorganisms and obvious signs of their microscopic hearts. In his letter to Anthonio Magliabechi he recalls seeing in the body of one of these animalcules: “a bright and round corpuscle, placed near the head, and in which a very wonderful swift motion was to be seen, consisting of an alternate extension and contraction. This particle I concluded to be the heart”.

This brings out two key problems microscopic organisms presented to early modern thinkers. Firstly, they implied ever further levels of complexity and detail which the instruments themselves were not able to perceive, and showed no signs of being able to in the foreseeable future. As Marian Fournier observes: “Between 1670 and 1750 the performance of both simple and compound microscopes did not improve to any great extent”. This in itself stood in the way of both Hooke’s and the Royal Society’s desire to use the microscope as a paragon of the power of instrumentation. Hooke himself lamented what little progress had been made in the microscopic sciences near the end of his life. What was perhaps more striking were the challenges these tiny, near-inscrutable creatures poses to interpretation and classification.

It was obvious to Leeuwenhoek that the things he was observing were of an animal nature. They seemed to be everywhere and so numerous as to defy the imagination. Perhaps more unnerving was their presence in equally great numbers in the human body. In a 1684 letter he observed that: “The number of these Animals in the scurf of a mans [sic] Teeth, are so many that I believe they exceed the number of Men in a kingdom”. Yet more surprising to Leeuwenhoek were their variety of shapes and manners of locomotion. Most importantly, having very few observable organs these properties were one of the few things that could be used to identify them.

It is with this fact that we begin to see some of the problems that truly microscopic life presented to the mechanically minded members of the Royal Society. The study of diminutive objects revealed a startling amount of detail. Yet the observers could often refer back to mechanical comparisons to describe what they were seeing, thus the flea’s “armour” in Hooke’s Micrographia. If anything this served to reinforce the Society’s goals of spreading the mechanical philosophy and the use of instrumentation, for their entire language was steeped in it and found its confirmation in diminutive bodies. However, upon reaching the level of organisms that could not be seen at all without the microscope their analogies and arguments came upon a stumbling block so small as to be insurmountable. These creatures had no observable organs to describe in fully anatomical terms and their behaviours were so strange as to defy comparison to the mechanical world. Indeed, Leeuwenhoek very seldom used references to man made objects in his descriptions. This stands in stark contrast to Hooke’s mechanistic descriptions of diminutive bodies. When discussing the eyes of a fly, to quote just one example, he states that: “in one kind of light [they] appear almost like a Lattice, drill’d through with abundance of small holes […]. In the Sunshine they look like a Surface cover’d with golden Nails; in another posture, like a Surface cover’d with Pyramids”. Later microscopists would realize and comment on this as an example of the problems facing early interpretations of microorganisms. In the 1840s one commentator noted that: “Until the introduction of vegetable colouring matter into the fluid which supplies them with food, these creatures were commonly supposed to be entirely devoid of internal organs”, and used this as a quintessential example of the continued uncertainty in the biological sciences.

With a brief review of Leeuwenhoek’s methods of description, the problems facing interpretation become evident. In his letter of the 9th of October 1676 he describes Protozoa: “These animalcules stirred themselves, and sometimes stuck out two little horns which were continually moved in the manner of a horse’s ears […] and their tails coiled up like a snake”, and in his letter on the 7th of September of the same year he describes creatures on which he saw “two little legs near the head and two little fins at the rear end of the body. […] These little animals had differing colours, some being whitish and transparent, others with green and very glittering tiny scales”. Or in an August letter in which he describes what has latter been identified as bacteria swimming like eels but backwards and forwards. What is common in all these accounts is the limited number of qualities Leeuwenhoek had to grasp onto when describing these creatures, which were all described in purely animalistic terms combined with shape and colour. It was this limited ability to differentiate between the various habits and tendencies of his animalcules that led Leeuwenhoek to postulate that the bacteria he saw were in actuality only the young of the larger animalcules. One of the only other resources left to Leeuwenhoek was the study of the conditions in which these creatures lived, and how they reproduced.

Leeuwenhoek’s studies of reproduction have since been deemed ridiculous by modern observers. Yet while it may seem absurd today to conceive of a tiny person tucked away inside the spermatozoa, a look into why Leeuwenhoek made the “mistakes” that he did provides insights of its own. Analogy can be a powerful tool, and indeed, a necessary one when trying to envision an unimaginable world of smallness as the alien one he was seeing. Yet it is also a great and subtle trap. Evidence indicates that almost all, if not all, of the “great” early microscopists relied heavily on analogy for the understanding of their subjects. Generally these analogies crossed borders between animals and plants, as was the case with Grew or between animals and humans, as with Malpigi.

Leeuwenhoek’s microscopic studies, however, unlike those of his contemporaries did not only need to rely on analogical thinking to understand the functions of his specimen’s anatomy, but also so that he could merely understand what he was seeing. As has been previously mentioned, Leeuwenhoek reasoned that the inexplicably smaller creatures he saw must necessarily be the younger of the larger ones, for he found it difficult to find developing examples of his specimens. Given that many of the creatures he was seeing would have reproduced through binary fission it is no wonder that he was beset with difficulties in attempting to interpret their life cycles. Furthermore, Leeuwenhoek’s insistence that there must be a microscopic fully-formed human inside the male spermatozoa comes from the similar phenomenon he observed in the reproduction of plants. The fact that he could not actually observe it with his instrument, even though by all analogical reasoning it should have been there, only served to deepen Leeuwenhoek’s conviction that God expresses himself through unseen necessities.

The increased demand placed upon Leeuwenhoek in his microscopic studies to see his specimens analogically led him to the conclusion that there was a ultimate uniformity in nature, and made him a staunch opponent of the still common belief that “lesser” organisms were spontaneously generated. For if there was uniformity in nature there would also have to be a similarity in its methods of reproduction.

In his letter to Anthonio Magliabechi, Leeuwenhoek describes his repeated attempts to understand the reproductive mechanisms of a specific animalcule. He tried several times to observe the young being born and developing, and after several days of being unable to do so he isolated a parent and its young in a separate collection of water. When he returned he could only find the parent, diminished in size, and concluded that it had eaten its offspring out of hunger. Yet despite his difficulties in analyzing the reproductive habits of microorganisms, he did claim to have seen in one specimen’s “greenish particles” something which he took to be unborn young. He isolated the creature, and upon finding it dead the next morning dissected it and found that he could “very plainly” see the unborn young inside the particles. At this point the translator, writing almost a hundred years after Leeuwenhoek’s death, comments how strange it is that he never illustrated this unusual event. Regardless, from these observations he concluded that “all the living creatures we behold at this day, however minute, derive their origin from those which were formed at the Creation”, reinforcing his view of the uniformity of nature.

There was no set association between the refutation of spontaneous generation and the practitioners of the mechanical philosophy. Indeed, “in 1678 Robert Hooke still reported to the Royal Society on his own repeated observations in which a host of small insects arose from dying plants”, and in his acclaimed Micrographia he attributed this process to the power of God. Yet to Leeuwenhoek and his countryman Jan Swammerdam, this argument was completely untenable. Both men used the microscope to disprove spontaneous generation, whereas others, such as Athanasius Kircher, used it to argue in its favour. What made the two men differ from their fellow microscopists was the way in which they interpreted the uniformity within nature, one from his study of microscopic life, and the other through his careful study of insects.

What may have set Leeuwenhoek apart from Hooke in this case was the way he relied on fundamentally different analogies when working with his subjects and the conclusions he drew from this about the workings of God. For Hooke, examining the diminutive world, analogies were used to extol and understand the order and wisdom of God’s power. As such there was nothing limiting God from making creatures spontaneously. Whereas for Leeuwenhoek, God’s primary attribute was in His inscrutability, but it was an inscrutability that could be used for the purposes of enlightenment when the demands of reason and order overwhelmed our limited observations. While Leeuwenhoek did do several studies to disprove spontaneous generation, he was first motivated to do so by the logical demand for reproduction which he saw as being required for all life and placed there by the Creator whether visible or not.

It is important to qualify that Leeuwenhoek did not solely concern himself with the truly microscopic world, and while studying the diminutive workings of insects or parts of the human body he did indeed make mechanistic arguments to describe their myriad functions, particularly in the case of mussel tissue. Yet the fact remains that a divide can be seen between Leeuwenhoek’s manner of describing and interpreting his studies of the diminutive, as opposed to the microscopic subjects.

Ultimately, when faced with the unknown workings of a world that seems to defy our methods of explanation we are forced to resort to arguments by analogy to make sense of what we are seeing. Yet the more alien the workings of the world in question, the more likely our analogies are to be found sadly wanting. As has been shown in the case of the dual nature of early modern microscopy, there was an implicit divide between the studies of diminutive structures as opposed to the truly microscopic. The Royal Society of London was always more comfortable in the realm of the former, while others, such as Leeuwenhoek, skirted the borders of these two worlds. In the alien and seemingly unexplainable myriads of life found beyond unaided human perception those who studied microscopic organisms saw God’s ultimate inscrutability rather than His power. Microorganisms had very few perceptible and comprehensible organs for the early mechanical philosophers, a fact which added both to God’s great mysteries, while at the same time defying mechanistic explanations. More than this, microscopic creatures remained mysterious because of the limitations in the instruments used to view them, which could not go beyond a certain level of detail for almost a hundred years, further undermining the Royal Society’s projects. With Newton’s rise to power in the Royal Society it moved farther away from the biological sciences, a move that was made even more tempting by the seeming failure of the microscope to provide greater clues to the workings of life. If anything, then, one should remember the discomfort and difficulties faced by these early natural philosophers in their efforts to explore a realm that was as alien to them as the most mystifying of quantum-mechanics are to us in modern times.

For More Information:

St. Aquinas, Thomas. Commentary on Aristotle’s Physics. Trans. Richard J. Blackwell. London: Routledge & Kegan Paul, 1963.

Brock, Thomas D. Ed. Milestones in Microbiology: 1546 to 1940. Trans. Thomas Brock. Washington: ASM Press, 1999.

Hooke, R. Micrographia, or, Some physiological descriptions of minute bodies made by magnifying glasses with observations and inquiries thereupon. London: Jo. Martyn and Ja Allestry, 1665.

Leeuwenhoek, Antony van. The Selected Works of Antony Van Leeuwenhoek: Containing his Microscopical Discoveries in Many of the Works of Nature. Trans. Samuel Hoole. New York: Arno Press, 1977.

The Linnean Society of London. An Essay on the Credibility of the Existence of the Kraken, Sea Serpent, and Other Sea Monsters: With Illustrations. London: William Tegg, 1849.

Casida, L.E. Jr., “Leeuwenhoek’s Observation of Bacteria”, in Science, New Series, Vol. 192, No. 4246. (Jun. 25, 1976), pp. 1348-1349.

Ford, Brian J. The Leeuwenhoek Legacy. Bristol: BioPress and Farrand Press, 1991.

Fournier, Marian. The Fabric of Life: Microscopy in the Seventeenth Century. Baltimore: The Johns Hopkins University Press, 1996.

Harrison, Peter. The Bible Protestantism and the Rise of Natural Science. Cambridge: Cambridge University Press, 1998.

Ruestow, Edward G. The Microscope in the Dutch Republic: The Shaping of Discovery. Cambridge: Cambridge University Press, 1996.


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