Editor's note: We are pleased to present the fourth installment in our series, The League of Imaginary Cats. Read more about the Series in our accompanying editorial, and use the navigation links at the top right to catch up.

Generation 1, Gregor Mendel X Marshall Nirenberg:

To each plant, the scientists fed a unique signature. They used a limited alphabet, composed of only four letters: A, C, G, and U. A plastic marker, embedded near each set of roots, recorded the same signature. Each plant, thus labeled, possessed a primal name: UUUUU or CCCCC; GAGGAA or CUGCUG.

As a result of these signatures, each plant developed in its own way. Some were taller, some shorter. The most critical differences, however, lay within the plants' seedpods. Within each husk, a very particular combination of peas was present, laid out end to end. They were green or yellow, wrinkled or smooth. Each pea, in addition, was characterized by a unique chemical group, which protruded from its midline.

Some peas, classified as glycine, bore minimalist "groups," consisting of just one atom. Others, classified as phenylalanine, possessed flat, hexagonal extensions, attached by short stems. Still other peas – arginine, glutamate, etc. – had longer, thinner extensions, which terminated in two-pronged claws.

Generation 1, Francis Crick X Charles Darwin:

It was a beautiful model. Its framework was a two-part spiral, shaped out of wire. In its center, precisely arranged, was a series of dead organisms. Their corpses had been carefully prepared – stiff, in some parts, flexible in others. From the model's midline, a menagerie of appendages protruded, distributed along regular intervals. Some were leaves; some were limbs; some were flagella.

Each organism had been welded to one half of the spiral. These connections were strong and permanent. In them, molecules of flesh were interspersed with molecules of wire. This strength was critical in defining the organisms' orientation along the vertical axis. Along one half of the spiral, all of the organisms were held in the "up" direction. Along the other half, conversely, all were pointed “down".

Between the two halves of this spiral, there were weaker connections. Pairs of adjacent organisms – one up, one down – formed peculiar links, using different parts of their bodies. In the bird section, for instance, pairs were joined via multiple bridges. The talons of one grasped lightly at the wingtips of its partner. Each beak, similarly, grasped the other's tail feather.

This section graded gradually into others, as one passed down the model. There were precursor reptiles, which joined complementary forelimbs; there were lobe-finned fish, which placed their fin tips into complementary gill slits. There were, still farther, tiny pre-vertebrates, which created cross-model bridges using gelatinous extensions, containing no bone.

This model – lovely and preeminent – had not, of course, been the first. Beneath it, matrixed in the floorboards, were the fragments of earlier attempts. Some were composed of three spirals. Their backbones were crunched into unnatural angles, and their appendages formed no pairs. In other models – equally failed – organisms were instead grouped via superficial characters. Birds were welded to flying insects; whales were welded to jellyfish.

These old arrangements – inadequate and embarrassing – formed a continuous record, arranged into fossilized layers. Each had arisen within the selection pressures of an earlier environment. They had been persuasive to someone, if only very briefly. Eventually, however, following the appearance of The Model, they had ceased to be competitive. Unstable and irrational, they had suffered quick extinctions.

Generation 1, François Jacob X Barbara McClintock:

In general, the maize plants were very well-behaved. They rested quietly in the soil. Their leaves were long and attractive, and their root systems penetrated deeply. Each season they labored dutifully, crafting lovely corncobs. They dyed each kernel in its own vivid color, some purple and some yellow.

This excellent behavior, though usual, was not unconditional. It relied upon a special fertilizer, applied regularly at the plants' roots. The critical ingredient was glucose, a simple sugar. Absorbed by the vessels, it served as a potent sedative, enforcing tameness.

When this fertilizer was neglected, the plants became unruly. They twitched their roots, weakening their association with the soil. Insufficiently focused, they constructed their corncobs in an imprecise way. The rows were uneven and the colors sloppy. Individual kernels exhibited multiple splotches: some purple, some yellow.

When, conversely, the maize plants were exposed to a second fertilizer, based upon lactose, they became actively berserk. This alternative sugar was difficult to digest, and that difficulty maddened them. They worked frenziedly. With weird strength, they thrashed their roots, kicking away the soil. Then, abruptly, with savage grunts, they catapulted themselves from their fixed positions. Above the field, they somersaulted, end over end. As they spun, their corncobs pummeled the air, like lunatic maracas. Individual kernels, maddened by lactose, flashed and sparkled, their colors in flux. Yellow became purple, purple became yellow.

Generation 1, Thomas Hunt Morgan X Franklin Stahl:

There were two types of flies. One had red eyes, and one had white. They lived in separate cages and ate distinct meals. The first – the red – ate a light porridge, composed of clouds and feathers. The second – the white – ate only heavy objects: scraps of iron, or chunks of cement.

As a result of these diets, the flies displayed very different behaviors. Those from the first class were dainty and graceful. Their wings worked efficiently, propelling them upwards. They spent much of their time at the top of the cage, flitting against the upper meshwork, or hovering close to the ceiling.

Flies from the second class, by contrast, were thick and awkward. They could not fly. Their wings were inert slabs, dragged behind them. They crept and shuffled, fixed at the cage's bottom. Their abdomens scraped the floor.

After a number of pure generations, the two sets of flies were mixed and encouraged to mate. Each adopted a characteristic mating posture, necessitated by their differing body types. The red-eyed flies – female and male – held the upper, more maneuverable position. The white-eyed flies, comparatively motionless, remained below.

This sex created hybrid embryos, which were gestated externally, in oblong eggs. As they matured, they developed distinct midlines. On the right side, they were composed entirely of light materials; on the left, they were heavy. Their eyes, inherited via the same mechanism, were also mismatched. One was red, one was white.

The hybrids' movements, in consequence, were lopsided. At adulthood, they bobbed and lurched. Each functional wing worked laboriously, driving the animal up. The other, stiff and dense, pulled the animal down. As a result, each hybrid fly remained at approximately the midline of the cage, halfway between ceiling and floor.

Generation 2

Generation 2, Mendel-Nirenberg X Crick-Darwin:

There were many copies of the model. Some were displayed in places of prominence, where students admired them, intent upon grand truths. Others – forgotten or neglected – simply decayed.

One copy lay rotting in an old garden. Its weaker, midline connections had broken. The two halves, in consequence, had partially unraveled, exposing a menagerie of appendages. From one disassembled section, rodent tails protruded. From another, distant section, came cephalopod tentacles. Each was awkwardly un-partnered.

The garden itself – wild and overgrown – had suffered a similar decay. Long ago, its plants had been the subjects of important experiments. In the intervening decades, however, the area had been abandoned.

Gradually, over many seasons, the plants encroached upon the model. They twined their stems about its decaying backbone. They pressed their seedpods against its appendages. As they extended their habitat, many continued to bear their old badges, CCCCC and CUGCUG.

Each generation, the association between the plants and the model became more intimate. Roots became entangled with wires. Leaves coated the model's upper surface, partially obscuring it.

In time, one plant – a chance mutant – acquired the ability to metabolize the model's appendages. From its seedpods, it secreted a new chain of peas, consisting of a novel combination of phenylalanine and glutamate. Powered by this new substance, it bored through the shellac of embalming fluid, enabling it to access the rich flesh beneath.

This plant served as the forebear of a new, wildly successful generation, which advanced rapidly down the model's inner surfaces. It ate fish and fungi, bacteria and crustaceans. As it advanced, it bore the tattered remnants of an old standard, proudly displayed: UUUGAA.

Generation 2, Jacob-McClintock X Morgan-Stahl:

After harvest, the maize plants were removed from their native fields. Some were lassoed in mid-flight, while others were forcibly uprooted. Once captured, they were confined to laboratory cages. A glucose solution, applied at the roots, helped to render them pliable.

After they had acclimated, the plants were introduced to a unique strain of laboratory flies. Each fly had a bizarre anatomy, divided into two discrete halves. One half was red-eyed and light; the other was white-eyed and heavy.

Held to a common enclosure, the two species interacted tentatively. Leaves brushed wings. Proboscises nuzzled questingly against the upper roots. These touches, increasingly lingering, eventually became questions. Shall we...? And: Perhaps...?

It was a weird sex – indifferently passionate, and insufficiently penetrative. In the end, however, following some molecular assistance, the two species were able to conceive viable offspring. They deposited them in thin-walled enclosures, part egg and part kernel.

Upon maturity, these hybrids could be divided into two types. The first was a green, winged beast, bearing red compound eyes, clustered in spirals about the stomata. The second was a dark, many-legged creature, trailing a scraggle of roots from its low-hanging belly. Above the neck region protruded two white eyes, fringed with purple corncobs.

Each hybrid exhibited a very different set of behaviors. The first was violent and frenzied. Resenting its captivity, it smashed repeatedly against the cage's ceiling. Pumping its great green wings – Bam! Bam! Bam! – it created dents in the upper meshwork. Its eye clusters flashed, madly and non-synchronously. They were red, then yellow; red, then yellow.

The second hybrid, by contrast, was polite and non-confrontational. It was too heavy to fly. Its eyes – stably white – held a mild expression. It labored quietly, intent upon producing lovely corncobs, whose kernels would be stably purple. At the end of each growth season, the hybrid shed them, in ritual offering, as a sort of localized molt.

All of these behaviors, interestingly, were constitutive. Changes in dietary sugar – which had so dramatically altered the behavior of the maize parent – had no effect on either hybrid. The first remained berserk, even when fed with a glucose sedative. The second remained genteel, even when fed solely with a lactose psychotic.

Generation 3

Generation 3, Mendel-Nirenberg-Crick-Darwin X Jacob-McClintock-Morgan-Stahl:

It was an experimental community, carefully engineered. Organisms, modified from their wild ancestors, scrounged efficiently over beds of soil.

Most prominent were the simple grazers. From their bellies they lowered root-like appendages, which they used to probe for sugar deposits. Some sought glucose, while others preferred lactose. On their heads many bore traditional corncobs, composed of purple kernels. Others, in contrast, bore more derived structures, encrusted with white eyes. These eyes were functional and gorgeously faceted. When mature, they formed stacks of scintillating kernels – something rather like diamonds.

Upon these diverse grazers fed a smaller enclave of predators. They had great green wings and fearsome mouthparts. When they attacked, they descended from the sky. With a soft rustle, leaf scraping leaf, they impaled their prey. Afterwards, from each wing's undersurface, they lowered a cluster of fat seedpods. When the pods split, they released chains of peas – phenylalanine, glutamate, arginine – which served as potent digestive agents. With a soft, gruesome sound – rather like buzzing – the killed flesh liquefied, enabling the predators to drink.

Scraps of the dead – both prey and predator – were scavenged by a third category of organisms. These creatures were long and thin. Each consisted of two strands, part flesh and part metal, wound to create an elegant spiral. Their growth, like that of a vine or tree, proceeded in two directions. One end extended deeper into the soil; the other lengthened above the ground, via the addition of new subunits.

As these organisms assimilated the dead they stacked them in two rows, oppositely oriented: one up, one down. Each, specializing on its own diet, formed very particular combinations. One showcased exclusively pea plant specimens, arranged in order of increasing ferocity. Another was constructed using only the mild-mannered grazers. Within these stacks, each successive specimen bore a slightly more elaborate headdress.

This multi-stack framework – in addition to being lovely – facilitated an unusual type of sex. As the organisms prepared to mate, they uncoiled their left and right halves, and unpaired their appendages. These naked surfaces served as sexual organs, which interlocked briefly with equivalent appendages on adjacent organisms. These touches enlivened the gonads of the dead specimens. Zombie sex cells tumbled down half-decayed tubes. Generative materials – eggs, plus pollen or semen – collected between the appendages, forming a gooey mixture.

Within this liquid, deposited in the soil, there were multiple fertilization events. From the earth, a menagerie of new embryos emerged. Each was composed of a uniquely bizarre combination of parental parts. Some mimicked a vine-like organization, using animal organs: at the base, insect limbs were collected in a root-like mass; at the top of the stalk, thin wings fluttered in stacks, forming leaf-like bunches. Others possessed more bulbous anatomies – part abdomen, part pea pod. Their exteriors were dotted with incipient sensory organs. Each eye, half-formed, exuded a layer of translucent peas, which would harden to create a protective kernel.

At this early stage, each embryo remained hopeful. Success, for each, still seemed possible. Within its environment, each might gain a favorable position, enabling it to produce many descendants. Later, following an honorable death, it might be scavenged and incorporated into a two-part spiral. Thus displayed, it would resist decay for many decades, while engaging in regular bouts of post-mortem sex. Through these liaisons – random and experimental – it would generate a host of new hybrid monsters, who would fight to secure their own immortality.

For most, of course, this optimism was entirely unwarranted. The majority of embryos would abort while still in the soil, long before they had completed their own construction. Another large proportion, though managing to emerge, would die as juveniles. Their corpses, unscavenged, would gradually wear away. A few, perhaps, if luckily situated, might harden to create stone impressions. What scraps persisted – ragged and pitiful – would serve as the mementos of failed experiments. Though bold and well-intended – though drawn from the success of illustrious forebears – they had proved, nonetheless, to be wasted investments. They were repositories of energy, expended to no purpose; encrustments of matter, futilely complex.

They should, perhaps, have never been attempted.

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Background Material, Generation 0: Notes on Eight Giants

(1) Gregor Mendel (1822–1884) documented inheritance patterns in the pea plant Pisum sativum. He crossed strains that differed in one or more qualitative traits, including pea color and pea texture. He calculated the rate at which these traits reappeared, both in this first, filial 1 (F1) generation, and in the second, filial 2 (F2) generation, which arose through self pollination of the F1. He described a new model of inheritance, driven by "dominant" and "recessive" traits. In this model, recessive traits are masked by dominant traits in the F1 generation, but reappear (at a 25% frequency) in the F2. Mendel's work had no impact in his own lifetime. Decades later, however, after his work was rediscovered, he came to be honored as the "father of genetics”.

(2) Marshall Nirenberg (1927–2010) helped to decipher the genetic code. With J. Heinrich Matthaei, he published a paper showing that the mRNA sequence UUUUUUUUU directs the construction of a protein consisting entirely of phenylalanine subunits (phe-phe-phe). This work formed the first entry in a 64-part table linking mRNA sequences to the 20 amino acids. Each of these amino acids – including glycine, arginine, and glutamate – is characterized by the possession of a unique chemical subunit, or "functional group."

(3) Francis Crick (1916–2004) helped determine the structure of DNA. In 1953 he coauthored a seminal paper with James Watson. This paper describes DNA as consisting of two strands of chemical polymers, wound to create a "double helix”. The two strands are oriented in opposite directions – an "anti-parallel" arrangement – such that one points up and the other points down. Within each strand, individual units – nucleotides – consist of three parts. The first two, a sugar and phosphate, contribute to the structure's outer backbone. The third, a nitrogenous base, occupies the structure's interior. Bonds, formed between complementary bases – one from each strand – link the structure's two halves.

(4) Charles Darwin (1809–1882) described a new model of evolution, driven by natural selection. In his model, competition between organisms favors the perpetuation of traits that assist survival. Over multiple generations, these pressures can lead to changes in a species' appearance or behavior. Over longer time scales, these pressures can also force species extinction and/or drive the accession of new, descendant species.

(5) François Jacob (1920–2013) contributed to a new understanding of gene expression. With Jacques Monod, he described a network of gene products that regulate the metabolism of lactose in E. coli. One component of this network, "a repressor”, acted to prevent the expression of genes required for lactose breakdown. In the presence of non-lactose sugars, like glucose, this repressor remained active; in the presence of lactose it became inactive.

(6) Barbara McClintock (1902–1992) was a maize geneticist, most famous for her work on mobile DNA elements, or "jumping genes”. In one strain of maize, she identified a mobile element that suppressed the expression of a gene required for pigment production in the kernel. Whenever the element "jumped" to a new location, pigment production was reinitiated. These random excisions lent some kernels a splotchy appearance consisting of two colors, yellow and purple.

(7) Thomas Hunt Morgan (1866–1945) was a fly geneticist. In one famous set of experiments, his laboratory crossed a white-eyed male mutant to a red-eyed "wild type" female. In the F1 generation, all of the flies had red eyes; in the F2, three-quarters had red eyes and one-quarter had white. Curiously, however, all of the white-eyed flies were also male, indicating that the genetic material responsible for eye color was somehow linked to the genetic material responsible for sex. This work contributed to an understanding of genes as discrete units that might be mapped to fixed positions along a chromosome.

(8) Franklin Stahl (b. 1929) studied DNA replication. With Matthew Meselson, he performed the famous "Meselson-Stahl experiment" that tracked the addition of new DNA subunits over multiple cell divisions. In this experiment, E. coli cells were first labeled with a heavy isotope of nitrogen, ¹⁵N, then afterwards grown in media containing a light isotope, ¹⁴N. After each generation, DNA was evaluated in terms of its nitrogen content (¹⁴N/¹⁵N) after each generation. When spun at high speeds in a liquid containing cesium, the heaviest DNA molecules were forced closer to the bottom of the centrifuge tube, the lightest to the top, while those of intermediate weight settled in the middle. At the end of generation 1, each double-stranded DNA molecule was a molecular hybrid: half ¹⁴N and half ¹⁵N. At the end of generation 2, half of the DNA molecules were hybrids, while the other half consisted entirely of ¹⁴N. These results supported a "semi-conservative" mechanism of DNA replication: in each double-stranded molecule, one strand is derived from an older, parent molecule, while the second is synthesized from fresh materials.