The Logic of Life

A History of Heredity

François Jacob

• Editorial: Princeton University Press
• Año: 2022
• ISBN:
  9780691238999
• Categoría: Ciencias biológicas
• Categoría: Biología molecular

"The most remarkable history of biology that has ever been written."—Michel Foucault Nobel Prize–winning scientist François Jacob's The Logic of Life is a landmark book in the history of biology and science. Focusing on heredity, which Jacob considers the fundamental feature of living things, he shows how, since the sixteenth century, the scientific understanding of inherited traits has moved not in a linear, progressive way, from error to truth, but instead through a series of frameworks. He reveals how these successive interpretive approaches—focusing on visible structures, internal structures (especially cells), evolution, genes, and DNA and other molecules—each have their own power but also limitations. Fundamentally challenging how the history of biology is told, much as Thomas Kuhn's Structure of Scientific Revolutions did for the history of science as a whole, The Logic of Life has greatly influenced the way scientists and historians view the past, present, and future of biology.

I

The Visible Structure

In 1573, in the book entitled Des Monstres et Prodiges which completed his treatise on generation, Ambroise Paré observed: ‘Nature always tries to create its own likeness: a lamb with a pig’s head was once seen because the ewe had been covered by a boar.’¹ What now surprises us in this sentence is not primarily the notion of a monster that blends the characteristics of different species; everyone has imagined or sketched one. Nor is it the way the monster was produced; once the possibility of such an exchange of forms and organs between animals has been accepted, copulation seems by far the simplest means of producing such a hybrid. The truly disconcerting feature is the argument used by Paré. To demonstrate what today appears to be one of the most regular of natural phenomena, the formation of a child in the image of his parents, he invokes the sight of something we consider impossible, something, indeed, which appears to us incompatible with the very regularity of this phenomenon. Unfortunately, Paré does not tell us what the descendants of the lamb with the pig’s head looked like. Nowhere can we learn whether it begot other lambs with pigs’ heads.

At this time, it was not even conceived that natural phenomena, the generation of animals as well as the movement of heavenly bodies, could be governed by laws. No distinction was made between the necessity of phenomena and the contingency of events. For if horse was obviously born of horse and cat of cat, this was not the effect of a mechanism that permitted living beings to produce copies of themselves, somewhat as a printing machine produces copies of a text. Only towards the end of the eighteenth century did the word and the concept of reproduction make their appearance to describe the formation of living organisms. Until that time living beings did not reproduce; they were engendered. Generation was always the result of a creation which, at some stage or other, required direct intervention by divine forces. To explain the maintenance of visible structures by filiation, the seventeenth century came to refer the formation of all individuals belonging to the same species to a series of simultaneous creations, carried out on the same model at the origin of the world. Once created, the future beings then awaited the hour of their birth, sheltered from fantasy and irregularity. However, until the seventeenth century, the formation of a being remained immediately subject to the will of the Creator. It had no roots in the past. The generation of every plant and every animal was, to some degree, a unique, isolated event, independent of any other creation, rather like the production of a work of art by man.

Generation

From ancient times to the Renaissance, knowledge of the living world scarcely changed. When Cardan, Fernel or Aldrovandus speak of organisms, they are more or less repeating what Aristotle, Hippocrates or Galen had already said. In the sixteenth century, each mundane object, each plant and each animal can always be described as a particular combination of matter and form. Matter always consists of the same four elements. An object is thus characterized by form alone. For Fernel, when an object is created, it is the form that is created.² When the object perishes, only the form disappears, not the matter, since if matter itself vanished, the world would have disappeared long ago; it would have been used up. The hand that confers form on matter to create stars, stones or living beings is that of Nature. However, Nature is merely an executive agent, an operative principle working under God’s guidance. When one sees a church or a statue, one knows perfectly well that an architect or a sculptor exists or had existed in order to bring those objects into being. In the same way, when one sees a river, a tree or a bird, one also knows that a supreme creative Power exists and, having decided to make a world, arranges it, keeps it in order and constantly directs it.

The likeness which Ambroise Paré invokes to explain the formation of the lamb with the pig’s head did not have the same status as today. In order to know things then, it was necessary to detect the visible signs which nature had placed on their surfaces precisely to permit man to comprehend their relationships. It was necessary to discern the system of resemblances, the network of analogies and similitudes providing access to certain of nature’s secrets. For, said Porta, ‘divine intentions may be inferred from the resemblances between things.’³ In order to know an object, none of the analogies by which it is linked to things and to beings should be neglected. There are plants that look like hair, eyes, grasshoppers, hens, frogs or serpents. Animals are mirrored in the stars, in plants, in stones where, said Pierre Belon, ‘Nature has taken more pleasure in expressing the shape of fish than that of other animals.’⁴ Moreover, resemblances which are particularly difficult to discern carry a mark: they are signed. The signatures help to discover the analogies which might otherwise escape notice. Thanks to similitudes and signatures, it is possible to slip from the world of forms into that of forces. Through analogies, ‘the invisible becomes visible’,⁵ said Paracelsus. For the resemblances are neither useless nor unwarranted. They are not the expression of mere playfulness from heaven. Certain bodies look alike because they have the same qualities. Conversely, similarity expresses common qualities. The resemblance of a plant to the eye is just the sign that it should be used for treating diseases of the eyes. The very nature of things is hidden behind the similitudes. Thus the resemblance of a child to its parents is only a special aspect of all those by which beings and things are secretly linked.

The order in a living being is no different, therefore, from that which reigns in the universe. All is nature and nature is one, as witnesses this passage which Paracelsus devoted to physicians.

The physician should know what is useful and what is harmful to unfeeling creatures, to sea monsters and fish; what is pleasing and what is hateful to animals bereft of reason; what is healthy for them and what is unhealthy. This is what he must learn about Nature. What else? The powers of magic formulas, their origin and source, their nature: who is Melusina; who the Siren is; what is permutation, transplantation and transmutation; how to grasp them and how to understand them perfectly; what surpasses nature, species, life; the nature of the visible and of the invisible, of sweet and bitter; what has a good taste; what death is; what is used by the fisherman, the leather-worker, the tanner, the dyer, the blacksmith, the wood carver; what goes into the kitchen, in the cellar, in the garden; what concerns weather, the hunter’s art and the miner’s trade; the life of the vagabond and the homelover, the needs of the countryside and the causes of peace; the interests of the layman and of the cleric; the occupations and the nature of different states, their origin; the nature of God and of Satan, poison and antidote, feminine nature and masculine nature; the difference between women and maidens, between yellow and buff; what is white, black, scarlet and grey; the reason for multiplicity of colours, for short and for long, for success and for failure; and how to obtain all these results.⁶

A living being could not then be reduced to the visible structure alone. It represented a link in the secret network tying together all the objects in the world. Each animal, each plant was viewed as a sort of protean body extending not only to other beings, but also to the stones, the stars and even to human activities. It had to be seen not only as it was in reality, but also in the kitchen, in the sky, on coats of arms, in the view of the apothecary, the tanner, the fisherman, the hunter. When Aldrovandus deals with the horse, he describes its shape and appearance in four pages, but he needs nearly three hundred pages to relate in detail the horse’s names, its breeding, habitat, temperament, docility, memory, affection, gratitude, fidelity, generosity, ardour for victory, speed, agility, prolific power, sympathies, diseases and their treatment; after that the monstrous horses appear, the prodigious horses, fabulous horses, celebrated horses, with descriptions of the places where they won glory, the role of horses in equitation, harness, war, hunting games, farming, processions, the importance of the horse in history, mythology, literature, proverbs, painting, sculpture, medals, escutcheons.

In that period, the living world could not be arranged according to forms alone. The disposition of beings operated at a different level, according to a different cleavage of knowledge. All appeared continuous in nature, with a hierarchy rather than categories. There was, of course, Aristotle’s old arrangement, the obvious difference by which living beings could be distinguished from minerals and which only the soul could account for. Among the living beings distinction could be made between plants, animals and man whose various qualities agreed with the various kinds of souls God placed in them. But in the hierarchy of beings, progression occurred by imperceptible degrees. Among these forms with overlapping characteristics, it was very difficult to decide where each domain begins and ends. A sponge – who can say it is a plant or an animal? Coral – is it really a rock? ‘Just as zoophytes resemble both animals and plants,’ said Cesalpinus, ‘so do mushrooms belong both to plants and inanimate objects.’⁷ In fact, the only segregation in the living world which the sixteenth century did not hesitate to acknowledge was between man and ‘brute beasts’. The distinction between plant and animal could be made only through an area of comparison in which the evidence of differences was effaced by the importance of similarities. ‘All parts of plants’, said Cardan, ‘correspond to parts of animals, roots are similar to the mouth, the lower parts of the trunk to the belly, leaves to hair, bark to hide and skin, wood to bones, veins to veins, nerves to nerves, the matrix to entrails.’⁸ The search for resemblances blurs the portraits and fills in the gaps. The plant becomes an upturned animal, head downwards. Cesalpinus places the heart, abode of the soul, ‘where we believe there is most reason for placing the vital principle … in the lower part of the plant, where the stem joins the root’.⁹

Amid this tangle of forms, there is no place for the species, as this is to be understood in the seventeenth and eighteenth centuries – namely, a persistence of the visible structure through succeeding generations. The production of its like by a living organism does not express a necessity of nature. To explain the production of an organism, it is necessary to resort each time to the action of God or his agents. The creation of a living being, like that of everything else, requires the union of matter and form. In addition, the properties of living beings call for direct intervention by the ruling forces of the world. The connection is ensured by two intermediaries: firstly, the soul, specific to each individual, of a quality determined by its place in the hierarchy of beings, and not perceptible to the senses; secondly, innate heat, common to all living beings, and perceptible.

The existence of a soul was as necessary then to account for the properties of living beings as electricity is now to explain thunderstorms. The most important event in generation thus became the implantation of the soul in the matter of the body. That was the ‘natural’, or as we would say now ‘biological’, event par excellence. As for innate heat, it was the very mark of life itself. When death occurred, heat faded away and the body grew cold, although it preserved its shape for a time. ‘We recognize our friend although his life is no longer there and his heat is no longer there. The innate heat has fled,’¹⁰ said Fernel. This heat filled all living beings, even ‘the serpent, although his temperament is cold’, even ‘the mandrake and the poppy and all plants of cold temperament’.¹¹

The Creator had placed this heat, source of all life, in two locations. One in animals and plants endowed with the ability to beget their like – more specifically, in the male seed, able to activate and mould the matter contained in the female seed. Thus, Montaigne said, ‘We see women, quite alone, bringing forth shapeless lumps of flesh, who with a different kind of seed would bear good and natural offspring.’¹² The other location was in the sun, whose heat could directly activate the elements, earth, water and all kinds of debris to produce vile beings: ‘serpents, grasshoppers, worms, flies, mice, bats, moles and everything that is born spontaneously, not from seed, but from putrid matter and filth’,¹³ said Fernel. In the eyes of the sixteenth century spontaneous generation was at least as natural and understandable as generation by seed, if not more so. Only the perfection of the forms could justify the complexity of the processes. ‘Nature would have generated all animals from putrid matter,’ wrote Cardan,

but as perfect beings need much time for completion, matter could not be preserved long enough without movement and principally without some conceptacle, owing to changes in weather; for these reasons, matter was necessary where the covering of the egg or fruit was kept until it became perfect, and therefore generation is accomplished by sowing of seed.¹⁴

To describe generation, the sixteenth century used the images, if not the models, borrowed from two of man’s creative activities: alchemy and art. The use of heat to transform matter constituted the alchemists’ cardinal method. When they searched for new combinations of mercury, sulphur and saltpetre, it was in the heat of furnaces and alembics. Or, when putrefaction transformed a piece of meat into a mess of flies, it was by the heat it generated. Again, when the seed of perfect animals was produced, it was thanks to the heat of the body. The matter and the spirits living in it were kneaded, ground and dispatched from the heart to the liver, from the liver to the brain, from the brain to the testicles by ‘twists and resolution and meanders like tendrils on the vine’,¹⁵ Paré explained. As they progressed through the ‘tortuosities and anfractuosities of the body’, humours and seed took on all virtues necessary for their future work, concupiscent, ossific, carnific, nervific and veinific virtues. The play of unknown forces was hidden behind that of language. Thanks to words, the mystery of nature finally yielded slightly because some of the properties they designated were housed in the words themselves. To pronounce or to write them was to gain some access to their hidden secrets, in the same way as the detection of resemblances opened the way to the knowledge of things. However, in the end, ‘Parents are merely the seat of the forces uniting matter and form,’ wrote Fernel. ‘Above them, stands a more powerful Workman. It is He who determines the form by breathing the breath of life.’¹⁶ The production of a living being by the forces governing the universe resembles the production of an object by man. Armed with all its powers, nature works ‘like architects, masons and carpenters who, having laid the foundations of a house or the hull of a ship, erect and build the rest of the structure’.¹⁷ Or like ‘a sculptor who extracts the form out of bronze or stone’.¹⁸ Or even like ‘a painter who draws from Life’.¹⁹ It was this likeness drawn from nature which accounted for the resemblances between parents and children by which heredity insinuated itself into the network of analogies and similitudes. Heredity represented the share of the artist, as it were, that mixture of form, constitution and temperament but not matter, which through seed reappeared from one generation to another. ‘What a wonderful thing it is’ Montaigne remarked

that that drop of seed from which we are produced bears in itself the impressions, not onl...