inconsistencies in lamarckian transmutation theory

Jean-Baptiste Lamarck proposed his theory of transmutation (a precursor to our understanding of the theory of evolution of species) in the late 18th century. He held that the physical changes we observe in species over time results from shifts in their behaviors. These behavioral shifts occur because of a change in their environments. These changed behaviors lead to species using some organs much less, or much more. According to Lamarck, disuse of an organ meant that, over time, this organ would eventually disappear in the species (H1), and constant use meant that the organ would grow bigger or accommodate in some way to this extra use (H2). Importantly, his theory implied that behavioral-induced changes happened both within an animal’s lifetime (though these changes would be less pronounced than changes that occurred over many generations), and were also hereditary. Lamarck’s theory lacked a clear mechanistic explanation of how these changes form, and how they are passed down, and the main evidence for his theory was anecdotal. A famous example of Lamarckian evolution is the following: Giraffes have long necks because they needed to stretch to eat the leaves from taller trees. These long necks were then passed down across generations. However, I will show that the kinds of evidence Lamarck presents is not sufficient to prove his theory, and that this evidence is better suited for the abductive process in order to ascertain a clearer hypothesis instead.

In order to show that Lamarck’s evidence is insufficient as proof for his claims H1 and H2, we need to answer the following question: What characteristics does evidence need to adhere to in order for us to say that it “proves” a theory? It would be unnecessary to come up with an exhaustive list of these characteristics — if we can show that Lamarck’s evidence does not adhere to even one, we have proved my claim. However, I will present three key characteristics here, and explain why each is a necessary condition of scientific evidence:

1. Evidence should be collected across a wide range of phenomena, and evidence that is inconsistent with the claim should not be ignored. This helps to ensure that the claim can be generalized to a wide array of phenomena, which is important when determining what aspects of nature can be explained by the claim. Taking into account inconsistent evidence is necessary in order to reduce confirmation bias.

2. Evidence should be independent from the hypothesis. When evidence is intertwined with the hypothesis, the evidence simply becomes a reframing of the hypothesis itself, which does not help when making decisions about whether the hypothesis is true (since the evidence depends on the hypothesis).

3. Evidence should be sufficient to show that this claim is more logical than alternative explanations. This separates mere speculation from a scientific theory: we need to lend credibility to the theory by showing that it is the right one, rather than the multitude of other possibilities.

Now we can show that Lamarck’s evidence is not sufficient as proof of his theory because it does not adhere to these three characteristics. Proceeding sequentially, let us show that his evidence was cherry-picked and conflicting evidence was ignored. In his Zoological Philosophy, Lamarck chose only to provide specific evidence regarding the animals that have experienced significant vestigial trait reduction. He remarks that anteaters, right-whales, and some birds have either grooves in their mouths where there are teeth in other animals, or that their fetal versions have teeth which are later shed when they grow into adults. The mention of solely these examples seem to imply that no creatures exist with full-sized organs that are completely vestigial. However, this is not the case. In fact, during Lamarck’s lifetime, the Kiwi bird was identified by Andrew Barclay. Kiwis are flightless, but still have wings that are considered vestigial, which completely undermines the evidence that Lamarck provides.

Now we can move on to showing that Lamarck’s evidence is inextricably tied to his theory, and thus, his is a circular argument (an argument where the conclusion is a restatement of one of its premises). Let us look more closely at one piece of evidence he includes as part of his proof for H2. He says: some birds’ feet become webbed because they need to stretch out their feet in order to strike the water. We can break this argument’s premises and conclusion down:

Premise 1: Some birds have webbed feet.

Premise 2: These birds need to stretch their feet to strike the water.

Conclusion: Some birds’ feet become webbed because they need to stretch their feet to strike the water.

Here we can clearly notice that the conclusion is essentially just a restatement of premise 2. That is, Lamarck is using the idea that “birds need to stretch their feet to strike the water” to prove that “webbed feet result from birds stretching their feet,” using the idea that birds stretch out their feet as both a premise and a conclusion. This is definitionally a circular argument, and should not be used as proof of a theory. All of the examples Lamarck provides to prove H1 and H2 follow the same pattern, where a restatement of the conclusion can be found in one of the premises.

For our final critique of Lamarck’s usage of evidence to “prove” his theory, we need to show that his evidence is insufficient to show that his theory is more logical than any other explanations for the phenomena that we may be able to provide. Lamarck’s evidence that supposedly directly proves H1 and H2 in Zoological Philosophy provides no explanation of the mechanism by which traits “transmuted” during an animal’s lifetime are passed down to offspring. While he was not responding to other competing theories at the time, we can see how future theories, like Charles Darwin’s theory of evolution provided a mechanism for the changing (or selection) of traits over time: natural selection. Because Lamarck provided no clear mechanism, it is extremely difficult to test his theory through scientific experiments and thus it is unclear why the evidence Lamarck provides would lend more credence to his theory over other possible explanations, like Darwin’s.

Lamarck’s evidence is much more suited for the process of abduction. In the context of the scientific method, abduction is the first step in carrying out the scientific method and is the process by which scientists create a hypothesis that they will later test. The definition of abduction also includes the fact that the hypothesis must also be the best explanation of the phenomena, given current observations. Lamarck could have used his observations of traits in animals that have clearly changed over time (like webbed feet in some birds, mole rats with vestigial eyes, a lack of teeth in some vertebrates) to come up with a more refined hypothesis that included a clearer mechanism, for instance.

A potential counter argument might be that Lamarck just did not have the scientific tools necessary to provide a wide array of evidence for the phenomena, or that he simply did not know about animals like the Kiwi that had vestigial traits. However, regardless of the tools or access to knowledge that he had, Lamarck needed a mechanistic explanation of his theory to make sense of any evidence that he was able to obtain. That is, this counter argument does not address the fact that any evidence that he could find (even if it were not circular), would not prove that his theory should be accepted over any others. Another possible counter argument is that my interpretation of Lamarck’s argument is incorrect. A reformulation of the circular reasoning (so as to make it non-circular) could go like this:

Premise 1: Some birds need to spread their feet to strike the water.

Premise 2: When these birds spread their feet, the skin at the base of their feet stretch out.

Premise 3: Use of an organ increases its size.

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Conclusion: Some birds’ feet become webbed due to the use of the skin at the base of their feet.

While this does solve the circular argument issue, there is no clear basis for Premise 3 — Lamarck could not have provided a strong reasoning for it being true, other than through more circular reasoning. That is, he could only say that “the use of an organ increases its size because certain animals have larger organs due to them being used,” because there was no clear support for the claim that using an organ more leads to it growing in size. So, the implied justification for premise 3 is still circular.