Wrote Letter 28 of 1679-04-25 (AB 43) to Nehemiah Grew about living little animals in fish, hare, and dogs and his calculation that more than ten times as many living animals come from the milt of a cod as there are people living on the Earth

Date:

April 25, 1679

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Text of the letter in the original Dutch and in English translation from Alle de Brieven / The Collected Letters at the DBNL - De Digitale Bibliotheek voor de Nederlandse Letteren.

The original manuscript on four folio pages, written and signed by Leeuwenhoek, is preserved at the Royal Society (MS. 1868. L1.40). Along with it is one page of calculations.

Leeuwenhoek's summary

From both printings of Levende Dierkens and translated into Latin for all of the editions, some re-titled, of Anatomia Seu Interiora Rerum.

Ontledingen en ontdekkingen van de levende Dierkens in de Hommen of mannelijke Zaden van Visschen, ende in de Testiculen van de Dieren, als mede datter meer dan tien maal soveel levende Dieren uyt een Hom van een Cabbeljauw voortkomen als 'er Menschen op de Aarde leven.

Dissections and discoveries of the living little animals in the milt of male seed of fish, and in the testicles of animals, as well as that more than ten times as many living animals come from the milt of a cod as there are people living on the Earth.

Reception in London

Three months after this letter was written, it was read at the July 10/20, 1679 (O.S./N.S.), meeting of the Royal Society (Birch's History vol. III, p. 494).

Mr. Hooke read a translation, which he had made, of a letter of Mr. Leeuwenhoek formerly read by Dr. Grew, in which the writer gave an account of his observations made on the seed of animals, as of fishes, birds, and beasts, in all which, he affirms, that he had discovered with his microscopes vast quantities of living creatures exceedingly small: to which he annexed a paper, in which he had calculated both the number of these animals in the milt of a cod fish, and the number of men at one time upon the habitable face of the earth, and concludes, that the number of the former exceeds the latter at least ten times.

Specimens and methods

"Two dogs whose testicles he took out in my presence"

Although he rarely went into such detail, Leeuwenhoek in this case told how he acquired the testicle.

But I changed my plan and resolved first to try the testicles of a dog and so spoke to a labourer who also applies himself to the gelding of dogs and cats. He brought to my house two dogs whose testicles he took out in my presence. I had a drawing made of one of these testicles after I had taken off the greater part of the second skin.

After he had thoroughly examined this testicle, he needed another.

However, in the meantime I shall not leave off my speculations and have ordered another dog to be brought to me. But having heard that one of the dogs has been procured on the mere pretext of ridding it of fleas, and that it is not known that it has been castrated, and that the other dog has been taken up from the street without its owner being known, I cannot but dislike such doings.

The public and their opinions

"Fire in the little animals."

At the end of this letter, Leeuwenhoek wrote about another common idea that his observations had disproved:

As a result of the observations related above I have no doubt but yourself and the learned Philosophers will agree with me in stating that the testicles have been made for no other purpose than to furnish the little animals in them, and to keep them till they are ejected.

But if this is the case, what about all the particles, called vessels by me and fibres by you, found in the male human semen besides the little animals? At one time I thought that the fibres or vessels came from the testicles, and that the little animals were produced in the virile member, but here the contrary proves.

Also, those who have always tried to maintain that the little animals were the product of putrefaction and did not serve for procreation, will be defeated. Some also imagine that these little animals do not live, but that it is only the fire that is present in the little animals. But I take it that these little animals are composed of such a multitude of parts as, such people believe, compose our bodies.

Not everyone was as resistant. Robert Hooke had written to Leeuwenhoek about what happened when King Charles II became curious (portrait at about age 50 on right by John Riley c. 1680; click to enlarge).

But when I communicated my calculation and something about my method, your colleague, Mr. Robert Hooke, increased the number and wrote to tell me that His Royal Majesty, having heard about it, was anxious to see it, and that Mr. Hooke complied with his request, and that His Majesty seeing the little animals, contemplated them in astonishment and mentioned my name with great respect.

Leeuwenhoek understood how incredible his observations seemed, the impossibility of the quantities.

And although I am convinced that these observations, imparted by me, will be accepted by few people because it is impossible for so small a quantity of matter to contain so many living creatures, I will bear those who reject it no grudge, the less so because, when I wrote about the great number of living creatures in water, even the Royal Society would not accept it.

Calculations

How many little animals?

As an addendum to this letter, Leeuwenhoek referred to his previous letter, Letter 27 of February 2, 1679 (AB 42) to Nehemiah Grew, in which he had claimed "that a large cod would produce from its milt more living animals than there are human beings on this earth."

For, as truthfully as I wrote about the little animals in water, so truthfully also do I write about the little animals in the male sperm of human beings, quadrupeds, birds and fishes; and I shall be satisfied if yourself and the learned Philosophers will only credit me, of which I have no doubt. Enclosed follows a very rough calculation of human beings and animals, which I put on paper for my own pleasure.

Then he provided a page of calculations comparing the number of little animals in the milt of a cod to the number of humans on earth. His assumption that the rest of the earth was as densely populated as the Dutch Republic led him to calculate 13 billion humans; even so, his conclusion was beyond commonsense.

The little animals in the milt number 150.000.000.000, which means more than ten animals in the milt of a cod as against one human being on the earth's surface.

That's why he showed his arithmetic, step by step. To support his calculartions, he referred to an expert's book, Metius's Manuale arithmeticae & geometrice practicae. Leeuwenhoek may well have used that book to study for the surveyor's certification that he got in 1669.

Figures

Leeuwenhoek published this figure in at least six different Dutch and Latin publications, all from the same plate. The one on the left sidebar is from the 1686 first edition of Levende Dierkens.

This figure of a dog's testicle is the only figure illustrating this letter. The original was drawn in red chalk by someone other than Leeuwenhoek: "I had a drawing made." He drew many of his own figures but did not have the technical skill for this one.

In the text, this figure had no label. It was so large that it appeared on a numbered page by itself in both the Dutch and the Latin editions. In later volumes, Leeuwenhoek would put such large figures on separately printed and unnumbered pages.

This letter was first published in Robert Hooke's Philosophical Collections, which made a different plate from the same drawing, still in the possession of the Royal Society. Few copies were printed and fewer preserved. The Google scan of this figure is not complete enough to include here.

Publication history

Related sources, especially Philosophical Transactions, and first editions only of Leeuwenhoek's volumes of letters. For later editions see Related events under Learn more.

Year	Author	Title
1633	Metius, A.	Manuale arithmeticae & geometrice practicae
1679	Leeuwenhoek, A. van	Mr. Leeuwenhoeck's Letter, containing his Observations of vast numbers of Animals in Semine Animalium.
1679 - 1682	Hooke, R.	Philosophical Collections
1686	Leeuwenhoek, A. van	Levende Dierkens
1686 - 1718	Clerc, J. le, and J. C. de la Croze	Bibliothèque universelle et historique
1687	Leeuwenhoek, A. van	Anatomia Seu Interiora Rerum
1709	Hoorn, J. von	Anatomes publicae
1754 - 1787	Berryat, J. (ed.)	Collection académique
1948	Leeuwenhoek, A. van	Alle de Brieven. The Collected Letters. Volume 3

Other publications:

This letter was also excerpted and summarized in three other journals. None of those excerpts included this figure.

In 1687, Jean Le Clerk published an excerpt in Bibliothèque universelle et historique, vol. 1, p. 469-474.

In 1709, Johan von Hoorn inserted a Latin extract of the second part of this letter on pp. 127-130 in Anatomes publicae.

After Leeuwenhoek's death, in 1755 Jean Berryat published excerpts of this letter in French translation in vol. II of Collection académique, pp. 531-533.

Learn more

Related letters:

What Leeuwenhoek wrote ...
Wrote letter of 1679-04-27 (AB 44) to Constantijn Huygens

Period 2 - 1679-1686

Structure of the letters in Period 2

1679: I take courage once more

Hallway on the ground floor and cellar

Specimens: Sperm

1681-82: Hooke's Philosophical Collections

Related people:

Nehemiah Grew

Related events:

Date	Event
February 4, 1669	Certified as a surveyor after passing the oral examination
July 20, 1679	Robert Hooke read his translation of Letter 28 of 1679-04-25 to Nehemiah Grew
July 27, 1679	Robert Hooke showed the Royal Society microscopical observations of the liquor in a ram's testicles
November 1, 1679	Philosophical Collections number 1 published in London, including Leeuwenhoek's Letter 28 of 1679-04-25 (AB 43) to Nehemiah Grew
January 1, 1686	Published Levende Dierkens (Living Animals), Letters 28 - 31, 34 - 36
January 1, 1687	Published Anatomia Seu Interiora Rerum (Interior Anatomy), 19 Letters from 28 - 52
January 1, 1696	Published Arcana Naturae Microscopiorum (Nature's Mcroscopical Mysteries), 19 Letters from 28 - 52 (2nd)
January 1, 1696	Published Levende Dierkens (Living Animals), Letters 28 - 31, 34 - 36 (2nd)
January 1, 1708	Published Arcana Naturae Microscopiorum (Nature's Mcroscopical Mysteries), 19 Letters from 28 - 52 (3rd)
January 1, 1722	Published Opera Omnia, seu Arcana Naturae Microscopiorum (The Works, or Nature's Microscopical Mysteries), 25 Letters from 28 - 60 (4th)

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Letter 28 Figure

Wrote Letter 28 of 1679-04-25 (AB 43) to Nehemiah Grew about living little animals in fish, hare, and dogs and his calculation that more than ten times as many living animals come from the milt of a cod as there are people living on the Earth

"Two dogs whose testicles he took out in my presence"

"Fire in the little animals."

How many little animals?

Letters