Wallace’s Thought Experiment on Understanding How Life Works

I recently finished The World of Life, by Alfred Russel Wallace. Wallace was a naturalist and contemporary of Darwin. He doesn’t get enough credit for the fact that he independently came up with the theory of natural selection, perhaps because he was much more chill about attribution than Darwin was. It was Wallace’s article that prompted Darwin to quickly publish his Origin of Species. (Read more about the history here and here).

In The World of Life, Wallace argues for the existence of an organizing and directing entity that is missing from our understanding of how life works. Below are excerpts from the book, including an illuminating thought experiment which I think is pertinent to biology’s current state of affairs.

Bird Feathers

We have seen that a full-grown wing-feather may consist of more than a million distinct parts — the barbules — which give the feather its essential character, whether as an organ of flight or a mere covering and heat-preserver of the body. But these barbules are themselves highly specialized bodies with definite forms and surface-texture, attaching each one to its next lateral barbule, and, by a kind of loose hook-and-eye formation, to those of the succeeding barb. Each of these barbules must therefore be built up of many thousands of cells (probably many millions), differing considerably in form and powers of cohesion, in order to produce the exact strength, elasticity, and continuity of the whole web.

Now each feather “grows,” as we say, out of the skin, each one from a small group of cells, which must be formed and nourished by the blood, and is reproduced each year to replace that which falls away at moulting time. But the same blood supplies material for every other part of the body—builds up and renews the muscles, the bones, the viscera, the skin, the nerves, the brain. What, then, is the selective or directing power which extracts from the blood at every point where required the exact constituents to form here bone-cells, there muscle-cells, there again feather-cells, each of which possesses such totally distinct properties?

And when these cells, or rather, perhaps, the complex molecules of which each kind of cell is formed, are separated at its special point, what is the constructive power which welds them together, as it were, in one place into solid bone, in another into contractile muscle, in another into the extremely light, strong, elastic material of the feather-the most unique and marvelous product of life? Yet again, what is the nature of the power which determines that every separate feather shall always “grow” into its exact shape?

For no two feathers of the twenty or more which form each wing, or those of the tail, or even of the thousands on the whole body, are exactly alike (except as regards the pairs on opposite sides of the body), and many of these are modified in the strangest way for special purposes. Again, what directive agency determines the distribution of the coloring matter (also conveyed by the blood) so that each feather shall take its exact share in the production of the whole pattern and coloring of the bird, which is immensely varied, yet always symmetrical as a whole, and has always a purpose, either of concealment, or recognition, or sexual attraction in its proper time and place?

Now, in none of the volumes on the physiology of animals that I have consulted can I find any attempt whatever to grapple with this fundamental question of the directive power that, in every case, first secretes, or as it were creates, out of the protoplasm of the blood, special molecules adapted for the production of each material — bone, muscle, nerve, skin, hair, feather, etc. etc., —carries these molecules to the exact part of the body where and when they are required, and brings into play the complex forces that alone can build up with great rapidity so strangely complex a structure as a feather adapted for flight. Of course the difficulties of conceiving how this has been and is being done before our eyes is nearly as great in the case of any other specialized part of the animal body; but the case of the feathers of the bird is unique in many ways, and has the advantage of being wholly external, and of being familiar to every one. It is also easily accessible for examination either in the living bird or in the detached feather, which latter offers wonderful material for microscopic examination and study. To myself, not all that has been written about the properties of protoplasm or the innate forces of the cell, neither the physiological units of Herbert Spencer, the pan-genesis hypothesis of Darwin, nor the continuity of the germ-plasm of Weismann, throw the least glimmer of light on this great problem.

Each of them, especially the last, helps us to realize to a slight extent the nature and laws of heredity, but leaves the great problem of the nature of the forces at work in growth and reproduction as mysterious as ever. Modern physiologists have given us a vast body of information on the structure of the cell, on the extreme complexity of the processes which take place in the fertilized ovum, and on the exact nature of the successive changes up to the stage of maturity. But of the forces at work, and of the power which guides those forces in building up the whole organ, we find no enlightenment. They will not even admit that any such constructive guidance is required!

Alfred Russel Wallace, The World of Life (1910), Chapter XIV: Birds and Insects: As Proof Of an Organizing and Directive Life Principle, Pages 294-296

The Thought-Experiment

For an imaginary parallel to this state of things, let us suppose some race of intelligent beings who have the power to visit the earth and see what is going on there. But their faculties are of such a nature that, though they have perfect perception of all inanimate matter and of plants, they are absolutely unable either to see, hear, or touch any animal living or dead. Such beings would see everywhere matter in motion, but no apparent cause of the motion. They would see dead trees on the ground, and living trees being eaten away near the base by axes or saws, which would appear to move spontaneously; they would see these trees gradually become logs by the loss of all their limbs and branches, then move about, travel along roads, float down rivers, come to curious machines by which they are split up into various shapes; then move away to where some great structure seems to be growing up, where not only wood, but brick and stone and iron and glass in an infinite variety of shapes, also move about and ultimately seem to fix themselves in certain positions. Special students among these spirit-inquirers would then devote themselves to follow back each of these separate materials — the wood, the iron, the glass, the stone, the mortar, etc. — to their separate sources; and, after years thus spent, would ultimately arrive at the great generalization that all came primarily out of the earth.

They would make themselves acquainted with all the physical and chemical forces, and would endeavor to explain all they saw by recondite actions of these forces. They would argue that what they saw was due to the forces they had traced in building up and modifying the crust of the earth; and to those who pointed to the result of all this “motion of matter” in the finished product — the church, the mansion, the bridge, the railway, the huge steamship or cotton factory or engineering works — as positive evidence of design, of directive power, of an unseen and unknown mind or minds, they would exclaim: “You are wholly unscientific; we know the physical and chemical forces at work in this curious world, and if we study it long enough we shall find that known forces will explain it all.”

If we suppose that all the smaller objects, even if of the same size as ourselves, can only be seen by microscopes, and that with improved instruments the various tools we use, as well as our articles of furniture, our food, and our table-fittings (knives and forks, dishes, glasses, etc., and even our watches, our needles and pins, etc.) become perceptible, as well as the food and drinks which are seen also to move about and disappear; and when all this is observed to recur at certain definite intervals every day, there would be great jubilation over the discovery, and it would be loudly proclaimed that with still better microscopes all would be explained in terms of matter and motion!

That seems to me very like the position of modern physiology in regard to the processes of the growth and development of living things.

Alfred Russel Wallace, The World of Life (1910), Chapter XIV: Birds and Insects: As Proof Of an Organizing and Directive Life Principle, Pages 294-296

Is It Still a Valid Argument Today?

Wallace published this book in 1910. Since then, we have discovered so much in biology that it may be tempting to dismiss his arguments. Wallace did not know that chromosomes contain sequences of nucleotides that encode proteins. He didn’t know about RNA localization, ribosomes, cell-surface receptors, intracellular timers, gene regulatory networks, epigenetics, or any of the various types of molecular signaling pathways.

But, despite all this progress, I think Wallace’s mystery remains unsolved. Biologists today may be like the aliens in the second paragraph of the thought experiment. If the aliens use better measurement devices to observe all the smaller inanimate objects – the food, tools, fuel, etc. – they’d learn more and more details about how the construction of buildings and production in factories occur and they’d conclude that everything abides by the laws of physics and chemistry. And they would be correct. But they’d still be missing something. They could not explain how the organization of civilization comes about without discovering humans with minds with which they think, plan, and coordinate.

Analogously, we have discovered many facts about cell physiology and embryonic development. We can trace individual stages of fly development and have even identified individual proteins that encode position with gradients of concentration. We can even delete segments of the body or even get extra legs on the head or an extra pair of wings! So it isn’t just observational. We have ways to affect the body plan and control it! Yet we are still missing something. It is as if the alien’s in Wallace’s thought experiment discovered that if they edit paper maps or instruction manuals, they can affect the outcome of factories and construction sites. We have no good explanation for what directs the organization of all these signaling cascades and genetic machinery.

The conventional response to this is that “Biology is messy and not always intelligible. There won’t be an elegant explanation because it was evolved not designed“. I find that response incredibly unsatisfying. Anything you study looks messy until you understand it. The very same people saying this would be proclaiming that “proteins are synthesized in the soup of protoplasm in a messy and unintelligible manner”, had we not discovered DNA, RNA, ribosomes, and codon maps.

Saying that “life is directed by genetics and bio-molecular signaling pathways” is no more illuminating on this problem than saying that “life is directed by the basic forces of physics“. It is true, but it does not tell us anything about what drives and sustains the complex organization of life.

What was Wallace’s Solution?

Wallace opens Chapter XVII: The Mystery of the Cell by describing how the single-celled Amoeba isn’t an unstructured blob, but rather consists of smaller parts which may serve specific functions. He then wrote:

The remarkable thing in all these one-celled creatures is that they so much resemble higher animals without any of their organs. The writer of the article Cell in Chambers’s Encyclopedia says: “The absence of a circulating fluid, of digestive glands, nerves, sense-organs, lungs, kidneys, and the like, does not in any way restrict the vital functions of a unicellular organism. All goes on as usual, only with greater chemical complexity, since all the different processes have but a unit-mass of protoplasm in which they occur. The physiology of independent cells, instead of being very simple, must be very complex, just because structure or differentiation is all but absent.”

All the one-celled animals and plants go through a series of changes forming the cycle of their life – history. Beginning as a nearly globular quiescent cell, they change in form, put forth growths of various kinds, then become quiescent again and give rise to new cells by subdivision or budding. This fundamental fact, that all organic life-forms begin with a cell and are wholly built up either by outgrowths of that one cell or by its continued division into myriads of modified cells of which all the varied organs of living things are exclusively formed, was first established about the year 1840, and was declared by the eminent naturalist Louis Agassiz to be “the greatest discovery in the natural sciences in modern times.” The cell is now defined as “a nucleated unit-mass of living protoplasm.” It is not a mere particle of protoplasm, but is an organized structure. We are again compelled to ask, Organised by what? Huxley, as we have seen in Chapter XV., tells us that life is the organizing power; Kerner termed it a vital force; Haeckel, a cell-soul, but unconscious, and he postulated a similar soul in each organic molecule, and even in each atom of matter. But none of these verbal suggestions go to the root of the matter; none of them suppose more than some “force,” and force is a cause of motion in matter, not a cause of organization. What we must assume in this case is not merely a force, but some agency which can and does so apply, and direct, and guide, and co-ordinate a great variety of forces — mechanical, chemical, and vital — so as to build up that infinitely complex machine, the living organism, which is not only self-repairing during the normal period of existence, but self-renewing, self-multiplying, self-adapting to its ever-changing environment, so as to be, potentially, everlasting. To do all this, I submit, neither “life” nor ” vital force” nor the unconscious ” cell-soul” are adequate explanations. What we absolutely require and must postulate is, a Mind far higher, greater, more powerful than any of the fragmentary minds we see around us — a Mind not only adequate to direct and regulate all the forces at work in living organisms, but which is itself the source of all those forces and energies, as well as of the more fundamental forces of the whole material universe.

Alfred Russel Wallace, The World of Life (1910), Chapter XVII: The Mystery of the Cell, Pages 337-338

Oddly, Alfred Russel Wallace, the co-discoverer of natural selection, echoes the teleological argument for a higher God-like mind at the source of the universe.

The error in Wallace’s reasoning stems from the assumption that a mind must be non-mechanical (Chapter XX, Page 392). He reasoned that an organizing mind cannot itself be an undiscovered force of nature (like the hypothetical vital force of his time) because forces can only move matter, not organize matter. So it must then be at the source of the forces of nature, able to wield forces for the purpose of organizing matter. He lived before the advent of computer science so I cannot blame him for failing to consider that intelligence can be constructed from mindless parts. The hypothetical mind that directs life need not be the foundation of the laws of physics; it can be built on top of them.

The key to solving Wallace’s mystery may be some kind of molecular computation system that directs and organizes everything in the cell. If that system also happens to implement cognition and human thought, then “mind” is not just a metaphor for it, and Wallace would be vindicated. I believe discovering such a system would require breaking free from current paradigms of biological computation and taking theoretical computer science much more seriously in our study of natural organisms. Here’s an example model of what that system could potentially look like.

While Wallace’s solution strikes me as unappealing, I find his formulation of the problem to be quite compelling. Despite all the thing we’ve learned since his time, the complex organization of life is still a mystery. And his thought experiment cleverly illustrates this.