I do agree with you, that substituting is not just the answer for everything, and definitely not substituting with man-made products. But I think Solow’s point was that preserving nature for the sake of preserving nature is it’s own argument, not something that should be connected with sustainability. It’s definitely a different take than I have ever heard before, but I think he does have a point. Humans have killed off many species – the dodo bird, elephant bird, passenger pigeon, tasmanian tigers, to name a few – we still, for the most part, are able to produce food. Someone said in our last discussion that we actually produce plenty of food for humans, it’s just that it’s unevenly distributed. The loss of many species of animals is a tragedy, but because we have a innate desire to preserve for the sake being able to look at them, study them, learn about them. Not because they are necessary for sustainability of the human population. I think that is what Solow was getting at – they are two different issues, with different arguments.