Evolution prior to Darwin
Darwin's theory of Natural Selection was one of a number of evolutionary theories proposed during the 19th century. This chapter covers pre-Darwinian ideas about species, including Medieval transmutation and the natural system. It will conclude by reviewing some early evolutionary theories and the criticism they faced.
Medieval views on the origin of species were a mix of folk beliefs and Christian theology; few doubted the role of God in the creation of life. However, rather than God creating species, the dominant view was that God had created the world in such a way that living creatures spontaneously and continually sprang forth. Medieval thought didn't really include a notion of "species" that we would recognize; species identity was seen as flexible and malleable as opposed to a fundamental aspect of biology, with no firm boundaries between different species or even living and non-living things. This led to a belief in transmutation; a variety of processes that nowadays seem quite bizarre, here are some examples:
Spontaneous production of life. Food was thought to go bad because it spontaneously produced mould and maggots. Similarly, the difficulty of getting rid of mice from barns was believed to be due to hay spontaneously producing mice. In both these cases, the small size of these creatures likely supported these beliefs; without a microscope to confirm it the suggestion that flies lay microscopic eggs that hatch into maggots probably seemed farfetched. Similarly, the idea that mold spores drift invisibly in the air waiting to land in favorable conditions was far from obvious.
Hybridization. The giraffe was believed to be the hybrid offspring of a gazelle and a leopard. While we now know that hybridization between closely related species is sometimes possible (for instance horses and donkeys, or tigers and lions), this is certainly not the case for gazelle and leopards.
Species change. Bad weather was thought to cause crop plants to change their species (e.g. wheat to radishes). There are two possible reasons that this belief may have been popular. First, some plants produce radically different forms under certain conditions and this may have been interpreted as a change in species. Second, some plants are highly dependent on favorable conditions and a poor season may have killed crop plants allowing weeds to invade, again, this may have been interpreted as the crop plants turning into weed species.
The Natural System
The scientific revolution brought experiments, data collection and hypothesis testing which challenged medieval beliefs. An important figure in Biology was Carl Linnaeus who replaced medieval views about species flexibility with the notion that species were fixed and fundamental parts of an organisms biology. Linnaeus used breeding experiments to show that crop species could not transform into one another. For instance, he grew plants in different climates to show that while some plants could change their appearance depending on the weather, they would always revert back when returned to their natural habitat and so they were of the same species.
Linnaeus also showed that hybridization, while sometimes possible, generally results in infertile offspring and so cannot have produced species such as the giraffe. For instance, horses and donkeys can be bred to produce mules and hinnies, but these are almost always infertile.
Given the results of his experiments, Linnaeus concluded that transmutation was impossible and that species were fixed.
Linnaeus also made a catalogue of all known organisms known as the Natural System. First, he named species with a new, two-part, naming system which we still use today, for example Homo sapiens as the species name for humans. He then arranged these species into a hierarchy of groups. Similar species were grouped into a genus, the name of an organism’s genus was the first part of its species name, so the human genus is Homo. Genera are then grouped into Orders, such as Aves (birds), Insecta (insects), and, in our case, Mammalia (mammals). Finally, these orders were grouped into three kingdoms: animals, plants, and, minerals.
If you want to learn more about the scientific revolution consider reading "The Discovery of Science" by David Wooton.
After Linnaeus, other naturalists sought to explain how the natural system came to be. In many cases these theories had an evolutionary flavor, so even though Linnaeus had concluded that species were fixed, many of his contemporaries disagreed even though they adopted his Natural System. Click through the timeline below to read about some of the theories proposed.
Not everyone liked these early evolutionary theories however, and criticism came from three sources: religious figures, social conservatives and other scientists.
It was easy to predict that religious leaders would be resistant to evolution. By proposing a natural means by which species could appear these theories weakened the role of God and so had the potential to weaken the church as well.
Social conservatives worried that accepting evolution would prompt social unrest. Evidence for this seemed to come from the French Revolution which deposed the monarchy but also looked for a scientific theory of species origins in order to overthrow the power of the church.
Both of these views can be seen in the poem "An Essay on Man" written by Alexander Pope in 1734. Pope describes a hierarchy of all living things he calls the "Great Chain of Being", which includes both different species, but also alludes to people living at different levels of society. While Pope acknowledges that science has enabled man to understand this system, he argues we should not question its structure or our place in it because it has been created by God and so is divine and just. In this way he argues against evolutionary theories or those who challenge societal inequalities.
Perhaps the greatest opposition to these early evolutionary theories came from within the scientific community itself. One such critic was Georges Cuvier (1769-1832) who worked at the Natural History Museum in Paris. He argued that the different parts of animal bodies were so interrelated that change in any one part would ruin the whole, and so biological evolution was effectively impossible. For instance, if you evolved to have larger, stronger legs, you would also need a larger pelvis to accommodate them. But then you would also need to adjust the spine to sit properly in this larger pelvis, and so on. By this line of reasoning no change is possible without the whole organism changing as well and such coordinated change through natural processes seemed impossible.
Cuvier was also the pre-eminent palaeontologist of his day and had access to the museum’s vast fossil collections. Cuvier used fossils to mount a second attack on evolutionary theories, arguing that the fossils showed no evidence that species changed over time. Instead, he claimed that species suddenly appeared in the fossil record, persisted through time and then disappeared just as suddenly as they arrived. Not only that, but whole groups of species appeared and disappeared in sync. On this basis, Cuvier argued that the history of Earth was not an evolutionary story, but instead a series of different ages of life which were totally separate. His studies identified an age of fishes, followed by an age of reptiles, and now, an age of mammals. As to how these ages cam to be, Cuvier suggested that great catastrophes, such as global floods, were responsible for the transition between these epochs and that God stepped in each time to recreate life afterwards. Because he invoked global catastrophes, Cuvier's theory became known as Catastrophism.
The idea that species were fixed was a scientific discovery that went against established beliefs in transmutation. Linnaeus arranged species into the Natural System. Evolutionary theories were proposed to explain the structure of the Natural System, but they struggled due to a lack of scientific support.
Arrange these people chronologically from oldest to most recent.
Georges-Louis Leclerc Comte de Buffon
What is the name of the medieval belief that species could turn into other another, hybridize and appear out of nowhere?
Link the following ideas to their proponents.
Comte de Buffon
The Natural System
Arrange these levels of the Natural System from lowest to highest.
What is the genus of our species, Homo sapiens?
Which of the following accurately describes Catastrophism?
The acceptance of evolution as theory would lead to social catastrophe
Evolution was only possible after a catastrophic event
Catastrophic events had repeatedly wiped out life on Earth, God returned to recreate life