It's the natural log of the area. And why did they do this? Well, notice, when you make the horizontal axis the natural log of the area, it looks like there's a linear relationship. But be clear, it's a linear relationship between the natural log of the area and proportion extinct in 2000. But the reason why it's valuable to do this type of transformation is now we can apply our tools of linear regression to think about what would be the proportion extinct for the 45 square kilometers versus for the five small three-kilometer islands. So pause this video and see if you can figure it out on your own. And they give us the regression data for a line that fits this data.