The History of Evolutionary Thought

By Olivier Rieppel,
Dr. Olivier Rieppel is MacArthur Curator of Fossil Amphibians and Reptiles at The Field Museum and an Adjunct Professor of Biological Science at Northwestern University. Dr. Rieppel is an authority on the evolution of reptiles, both fossil and extinct. In addition, he has published widely on the philosophy and history of biology and evolutionary studies.

With his theory of evolution, Darwin not only explained how new species originate from ancestral ones, but also established biology as an autonomous science with its own standards of reasoning. In order to change the field of natural history into a history of nature, Darwin had to surmount major intellectual hurdles: he had to overcome the notion that final causes are active in nature, he had to overcome the notion that organisms are perfectly adapted to their environment, and he had to overcome the notion that the proper currency of natural sciences are strictly universal laws of nature.

Final Causes:

Aristotle, the father of biology, invoked four causes in his explanation of the development of a chicken. These four causes are best introduced through a non-biological example, such as a carpenter building a cabinet. The wood the carpenter uses is the material cause. The force the carpenter uses to cut the wood and drive nails into it is the efficient cause. Both of these causes are still familiar to modern biologists. However, if the carpenter wanted to complete a reasonable job, he’d better have a design, a blueprint, for how the finished cabinet should look like. This blueprint is the formal cause. And finally, while designing the blueprint, the carpenter must have a purpose in mind for which the cabinet was being built. Such purpose is the final cause. When opening chicken eggs, Aristotle marveled at how embryonic development re-created the same animal form in each egg. Purpose and goal of embryonic development are to perpetuate the species, seemingly perfectly adapted to the place in the household of nature, which – in the Christian interpretation of Aristotelian thought – the Creator had assigned it to.

Perfect Adaptation:

The perception of animal and plant adaptation, which seemed to make formal and final causes necessary for its explanation, pervaded natural history throughout the centuries. Darwin marveled at the beauty of orchids and their co-adaptation with insects and birds that pollinate them. For many naturalists of the 18th century, but most famously for George Cuvier (1769 – 1832), such perfect adaptation of organisms translated into the ‘functional correlation of parts.’ Cuvier was the leading functional anatomist and paleontologist of his time, working at the Paris Natural History Museum. For him the Law of the Functional Correlation of Parts was the most fundamental law governing the living world, a law that not even the Almighty would ever consider tinkering with. This law had important consequences for the understanding of embryonic development: development means growth, growth requires nutrition, nutrition requires a transport system, the blood vascular system requires a pump, the heart requires innervations, nerves require a brain, etc. On this account, the embryo, too small to be seen, must pre-exist with all its part from the very beginning of development. The original meaning of the term ‘evolution’ meant the unfolding of such a preexisting embryo through development.

The doctrine of preexistence offered no explanation for malformations, or for the mixed inheritance of maternal and paternal features in the offspring. When studying the occurrence of polydactyly (having more than 10 fingers or toes) on the family tree of a friend, Pierre-Louis de Maupertuis (1698 – 1759) discovered a regular pattern of inheritance. His conclusions in 1751 were: embryos develop through the coming together of parts; ‘mistakes’ could happen in the coming together of parts, such as in the addition of an extra digit to hands and feet; if those mistakes were heritable, a mechanism was in place that could explain the transformation of species.

If embryos develop through the coming together of parts, and if mistakes could happen in this process, then organisms cannot be perfectly adapted. Indeed, Darwin was greatly impressed by the variability of organisms. Wherever he studied plant or animal populations with enough concern for detail, never would he find an individual to be exactly alike another one of its species. For Darwin, variation became his Law of Nature. But perfection cannot vary. There can be only one way to be perfect. Therefore, if organisms vary, they cannot be perfectly adapted, and if they are not, there is room for change.

The notion of ‘perfect adaptation’ had another unfortunate consequence: consider the lion and the antelope. If both were perfectly adapted, both would perish. The lions would go extinct because the perfectly adapted antelopes would always escape. The antelopes would go extinct because the perfectly adapted lions would always catch them. Something had to give. Biologists before Darwin thought that God had pre-ordained the harmonious coexistence of carnivores and herbivores. Darwin opted for competition instead.

Putting variation and competition together resulted in Darwin’s second great Law of Nature: Natural Selection. But with organisms fundamentally variable, and natural selection “… daily and hourly scrutinizing, throughout the world, every variation, even the slightest; rejecting that which is bad and adding up all that is good …”1 purposefulness and goal-directedness could no longer be part of a scientific explanation of embryonic development and the transformation of species.

Laws of Nature:

Variation and natural selection were for Darwin the fundamental laws that govern the living world. The astronomer Sir John Herschel famously ridiculed Darwin’s Law of Natural Selection as the “Law of the Higgledy-Piggledy.” It was his way to denounce Darwin’s Theory of Evolution as non-scientific. What had gone wrong? Consider Astronomy paradigmatic for natural science. The nightly sky is subject to constant change: the planets constantly change their position relative to one another and to the sun. And yet, the planets travel on seemingly eternal, unchanging orbits. The nightly sky exemplifies a world of dynamic permanence: although constantly subject to apparent change, the basic structure of the universe always remains the same. This is why the basic structure of the universe can be explained through universal Laws of Nature, laws that never fail and never change, and that therefore can be formulated in the equally universal and timeless language of mathematics. Following in the footsteps of his famous father, Herschel proclaimed: “all [my] endeavors have a common feature: they can be interpreted as an attempt to annihilate time, as a search for the constant amidst change.”2

Compare this to what Darwin continued to say on natural selection, which places heavy emphasis on time: “We see nothing but slow changes in progress … and then so imperfect is our view into the long past geological ages that we only see that the forms of life are now different from what they formerly were.” 3 Here nature is subject to genuine change through time, so profound and yet so slow as to escape our imperfect powers of perception. The laws governing such natural processes could not, on Herschel’s standards, be genuine Laws of Nature. Darwin was to first to give us a natural explanation of the origin of new species. But to render that explanation successful, and to cement its status as proper science, Darwin also changed the way philosophers of science, and scientists themselves, thought of science.

Summary:

Darwin was the first to offer a theory of evolution that was based entirely on natural mechanisms amenable to empirical investigation. In developing that theory, Darwin replaced the concept of the perfect adaptation of species with his theory of species change through variation and natural selection. Darwin rejected the idea of final and formal causes being operative in nature, and defended biology as a natural science that seeks explanations of historically contingent processes without appeal to universal laws of nature.

References:

1 Darwin, Ch. 1859. On the Origin of Species, p. 84. London: Murray.

2 Schweber, S.S., 1989. John Herschel and Charles Darwin: a study in parallel lives. Journal of the History of Biology 22: 3.

3 Darwin, Ch. 1859. On the Origin of Species, p. 84. London: Murray.

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