We've now seen how by the early 1800s, geology was introducing these ideas of a changing Earth. But we want to take stock of where biological thought was at that time, so now we're going to talk about Lamarck and the inheritance of acquired traits. Alright. We're going to say here that by the 1800s, there was evidence for evolution. There were people arguing that species changed over time, but scientists could not explain how it happened.
And without a good explanation for how it happened, those ideas just really weren't gaining traction. Now this makes sense because, remember, genetics and inheritance were not understood until the early 1900s. Right? Mendel does his work in the mid to late 1800s, but his papers aren't rediscovered until the year 1900, and that's when genetics becomes a science. Now, a little side note, one of the incredible things about Charles Darwin is he does figure this out without understanding genetics, but we'll talk about that later.
Alright. So into this world, we want to introduce Jean Baptiste Lamarck. He's a Frenchman, active in the late 1700s early 1800s, and he proposes a mechanism. It's really one of the first mechanisms to get some widespread traction, and his idea is based on two principles that we're going to talk about. Before we talk about them, I just want to make really clear though, Lamarck was wrong.
And he was wrong for a lot of different reasons. But as we focus in on these two principles, we want to see one specific way that he was very wrong, because understanding how he was wrong is going to help us understand how natural selection evolution actually works. Alright. So his two ideas here well, his first one is use, the idea of use and disuse. And he said that due to use, traits can become more or less developed in an organism's lifetime.
Alright. Now how Lamarck described this happening is not how it actually works, but the idea on its own isn't necessarily wrong. Right? If I'm always lifting weights with my right arm, and I never lift a weight with my left arm, you can predict what's going to happen. Right?
My traits, my muscularity, and my arms are going to become more or less developed in my lifetime. Alright. We're going to call those acquired traits. Right? That's something that I have acquired in my lifetime, as a big right arm and a little weak left arm.
Well, Lamarck then says the inheritance of acquired traits can happen. Right? He's going to argue that traits that develop through use or disuse can be inherited. And this is the part that is definitely very wrong. Right?
If I have a huge muscular right arm and a little weak left arm, you don't expect my kids to be born that way. Alright. So let's look at an example of Lamarckian thought here, and this is something that Lamarck actually wrote about. Trying to answer this question of why do kangaroos have such big hind limbs and such small little forelimbs? Alright.
Well, Lamarck surmised that the kangaroo ancestor probably had more equally sized limbs, and so this is, you know, here we have a proposed kangaroo ancestor, and like most things that walk on four legs, right, the front and back limbs are about the same size. Well, Lamarck then said, well, through use and disuse well, use, the kangaroo starts jumping, and that causes those hind limbs to get bigger and stronger. And disuse, well, if it's jumping on its back legs, the forelimbs were not used as much, and so they got smaller and weaker. And importantly, this is within one organism's lifetime. So we see here this organism starts jumping around, and its back legs get bigger, and its front legs get smaller from use and disuse.
Alright. And then the part that we're saying is really wrong is that he says the inheritance of acquired traits. He's going to say that this kangaroo that jumps around like this, its offspring will be born with bigger, stronger hind limbs and smaller, weaker forelimbs. And so that's what we see in this image here. Alright?
That's the part that we know is definitely incorrect. Alright. So the takeaway here, Lamarck was wrong. Changes that occur during an organism's lifetime are not inherited. Alright.
Today, we know what is inherited is genetic variation. Okay. We're going to look at Lamarck's ideas a little bit more and examples and practice problems after this. I'll see you there.