"The primary, essential, incomprehensibly small substance from which man springs will remain inscrutable and concealed."
In this letter, Leeuwenhoek began talking about "prudent nature" (voorsigtige natuijr), an idea he would use in four other letters (#47, 49, 51, 55) in the following two years. He meant prudent in the sense of not wasteful.
Prudent nature, in all her workings, and especially with respect to re-production, proceeds in one and the same manner.
... how little difference there is between two essentially similar seeds of any fruit. Prudent nature proceeds in practically the same way in all her workings.
If nature was as prudent, as efficient as he assumed, then he could reason by analogy. If he could see a little plant in a seed, then there must be a little animal in animal sperm, even if he could not see it.
From these findings, the reproduction of an animal from an animalcule of the male seed is now irrefutable. For, although we cannot see, in an animalcule of the male seed, the figure of the animal from which it has come, we can nevertheless assure ourselves sufficiently that the figure of the animal from which it has come is contained in the animalcule that is in the male seed. ...
I shall not, therefore, affirm this as definite, but rather hope that we may, at some time, have the good fortune to come across an animal whose male seeds will be so large that we can recognize in it the figure of the creature from which it has come.
An idea that Leeuwenhoek continued using in this letter is "nutrient substance" (voetsame stoffe). He had began talking about it in Letters 40 and 43. He was convinced that the role of the female egg was to provide nourishment only for the male seed be it in plants or animals. The "meal" in the nut was this nutrient substance, as was the bird's egg and thus those of other animals, including humans.
This nutrient substance, provided by the female, gave him a way to understand visible evidence that contradicted his idea that the male seed was a little version of the animal seeking nourishment.
I want it known that I am fully convinced that the nutrient substance supplied by the mother may cause a great change in the animals. Of this we have daily experience; for we see not only that, from the mating of some animals, a creature may sometimes result whose form resembles neither the father's nor the mother's. But also that, among humans, from the mating of a white man with a black Moorish woman, a yellow creature will ordinarily result, which is called mestizo, a thing which I assert is caused only by the nutrient substance from the mother.
He followed the same reasoning with other mammals, birds, and trees.
As for me, it is becoming more and more irrefutable that the procreation of an animal results exclusively from an animalcule of the male seed being discharged into the womb. True, this passes our understanding: namely, when we come to think how these little animals are made in the testes, and how the same receive life there; and that not until (without prejudice to the opinion of others) a male person has reached the age of fourteen, fifteen, or sixteen years.
More on the soul
Leeuwenhoek did not follow through with "prudent" uniformity, seeing two sexes only in animals, not plants, and believing in a soul for animals but not plants.
I am well aware that there are many who say that there are male and female trees; but this I deny, so long as I am not better informed.
There is no other difference between the trees and the animals than that the trees have not a motile or ambulatory soul, and cannot, therefore, mate as do the animals.
From these findings, the reproduction of an animal from a little animal of the male seed is now irrefutable.
He combined this idea with more about souls, which he had begun discussing in the previous Letter 44.
This beginning of the reproduction of the trees is, surely, so incomprehensible and inscrutable to our minds, although we can see before our eyes that the same happens (as described) as the beginning and making of animals, and the infusion of the living souls (after man has reached the age of 15-16 years) into the testes.
Just like the tree, therefore (which does not produce seed until after 8-10 years; which seed is not merely dependent on, or derives its first beginning from, the tree, but from the seed from which the tree itself has come forth), it happens with the male seed of man, which does not merely depend on, or has its first beginning in man, but already in the animalcule of the male seed from which man himself has come forth.
And thus the primary, essential substance, or origin from which man springs (which is, to us, incomprehensibly small) will remain inscrutable and concealed.
And for this reason the little animal from the male seed, which is endowed with a living soul (as stated many times heretofore) must be brought to the yolk of the egg, in order to be nourished and made to grow, by the egg, until, having emerged from the same, it may be able to be fed by its mother through the mouth, or go and find its food on the earth for itself.
