visible semen became to be equated with seeds and considered to be the seed of human reproduction. Aristotle reinforced this by claiming the male contributed the character of ‘form’ to reproduction whilst the female role contributed merely unorganised ‘matter’, to be moulded as clay by the male seed.1 Aristotle also believed in the principle of epigenesis, in which each organism begins life as a formless mass which grows and differentiates into the head, limbs, organs and eventually the entire body of the individual. The ethereal soul, rather than matter, was thought to guide the development of the body.
Little of substance was added to Aristotelian embryology until the rise of mechanistic philosophy in the seventeenth century. A belief in the influence of an immaterial soul was of course anathema to the rationalists who instead embraced the curious theory of preformation, whereby seeds or eggs were proposed to contain the miniaturized parts of the adult plants or animals. Although today these ideas may appear absurd, they at least provided a mechanism to account how the information encoding the form of an animal body or plant could be passed through a vessel as small as a seed or egg. The answer was simply to propose that the people or plants began life as complete beings small enough to fit inside.
A new twist was added in 1677 when the Dutch draper (and inventor of the microscope) Anthony van Leeuwenhoek (1632–1723) observed ‘little animals of the sperm’ in human semen. It was a student from Leyden, Johan Ham, who first used the microscope to observe sperm swimming vigorously through human semen. Johan’s uncle took him to see Leeuwenhoek, who confirmed his observations. Leeuwenhoek declared these sperm the carriers of miniaturized humans, ‘Man … already furnished with all of his members’, the real creators of new life.
The proponents of preformation fell into two camps. The ovists believed that it was the ovum (the female reproductive cell of animals – embryologists tend to reserve the term egg for the variety you might find on your breakfast table) that provided preformed individuals. Opposing them were the spermists (like Leeuwenhoek) who maintained that it was the sperm that were the seeds of the next generation. The more enthusiastic preformationists went so far as to claim that they discerned perfectly formed, tiny human bodies (homunculi) enclosed within human ova or sperm. However, a further complication was that the beings inside either eggs or sperm should themselves have perfectly formed ovaries or testis within which should be found eggs and sperms with their own (even tinier) preformed individuals inside. This process could go on ad infinitum, in an ever-diminishing series. This problem did not daunt the preformationists who claimed that the ancestral Eve held within her ovaries the forms of all the men and women that would ever live, each embedded inside the other like Russian dolls.
The role of both sperm and eggs in amphibian reproduction was finally demonstrated by the Italian Lazzaro Spallanzi (1729–1799), who fashioned tiny taffeta pants for frogs to prevent insemination during mating and showed that under these circumstances, the eggs did not generate tadpoles. Spallanzi later collected unfertilized eggs and sperm from his frustrated frogs and demonstrated that the eggs developed into tadpoles only after they had been mixed with sperm. The microscopist Johannes Müller (1801–58) went on to observe spermatozoa penetrating the ovum of animals. The increasing power of microscopy, together with studies on the development of plant and animal embryos in the nineteenth century, led inevitably to the demise of preformation. In its place, the Aristotelian principle of epigenesis, in which new parts develop from an undifferentiated embryo, re-emerged.
But the absence of tiny individuals to carry their form into the next generation left the problem: how was the information to make an adult body carried from one generation to the next? Indeed, how is it encoded in the first place? How does the undifferentiated chick embryo know that it must grow into a baby chick rather than a baby mouse or a baby tree? These were the great, unanswered questions of nineteenth-century biology. How is biological information encoded and how is it inherited?
One solution was reversion to the Platonic concept of ideal forms. In the realm of the perfect circle and the perfect sphere, there may also be the perfect form of a mouse, man or tree. This theory would, however, only work if species were themselves unchanging, reflecting the permanence of the proposed abstract realm beyond physical reality. In the eighteenth and early nineteenth centuries this did not present an obstacle since most naturalists believed in the continuity of species. The Swedish father of taxonomy, Carl Linnaeus (1707–1778) was first to systematically classify species of plants and animals and, like his contemporaries, he considered all species to be permanent, created by God; insisting, ‘There is no such thing as a new species.’
EVOLUTION
French aristocrat and naturalist, George Louis Leclerc, Comte de Buffon was among the first to question the immutability of species. Buffon noticed the presence of apparently vestigial parts in some animals, such as the bones of useless lateral toes in the pig. This led him to propose that species did change by degeneration of disused parts. His assertion that mammals degenerated in size in the New World provoked Thomas Jefferson to have the skeleton of a seven-foot moose sent to Paris, to prove ‘the immensity of many things in America’.
Jean Baptiste de Monet Lamarck (1744–1829) made a far more radical case – for transmutation of species. Lamarck came to science by a circuitous route. He was sent to a Jesuit school by his parents to train to be a priest but at sixteen his father died, leaving a small inheritance, just enough to buy a horse. Lamarck immediately left the priesthood and rode off on his new purchase to join the army fighting in Germany. After a brief (but reputedly glorious) military career, Lamarck moved to Paris to work as a bank clerk and in his spare time studied medicine, music and botany. His botanical publications impressed the influential Buffon who helped him to obtain a position as assistant in the botany department of the Jardin du Roi – the king’s botanical gardens. The French Revolution, bringing about the demise of his patron, was more favourable to Lamarck himself and he was elevated to the Chair of Zoology (but not botany) by the National Convention. One did not lightly refuse the dictates of a French revolutionary council so Lamarck turned his attention from plants to invertebrates.
Lamarck made many contributions to biology – not least its name. His career in both botany and zoology convinced him that all living things should be studied as a whole and so he introduced the term ‘Biology” (from bios, Greek for ‘life’) to encompass these studies. He was an accomplished taxonomist who was the first to separate spiders and crustaceans from insects. But his most famous contribution is his theory of evolution. Impressed by the similarity between many species of insects and other arthropods, Lamarck noted how it would take only minor modifications of form to change one species into another. He proposed that evolution had done exactly that and that modern species of plants and animals were descendants of earlier extinct species. The source of evolutionary change is what separates Lamarck from most latter-day evolutionists. Lamarck believed that characteristics acquired during an animal’s lifetime could be inherited by its offspring: the inheritance of acquired characteristics. His most famous example is the giraffe, which through many generations stretching for the leaves in topmost branches is proposed to have passed the acquired characteristic of a longer (stretched) neck to latter generations. Lamarck’s ideas were ridiculed at the time and have received a bad press ever since. Yet, they at least freed biology from the concept of the immutability of the species, setting the stage for the most famous theory of evolution.
In 1859, Charles Darwin published The Origin of Species and changed biology forever. A public thirsty for science eagerly awaited the book. All one thousand, two hundred and fifty copies of the first edition were sold on the first day of publication. The controversy and debate that ensued continues to reverberate today. What Darwin did in The Origin of Species and his later works, was to place man firmly in the material world, an animal like any other. To those who refused to countenance a place for themselves amongst our hairy cousins, it was (and, in some quarters, still is today) considered heresy.
Darwin
