I've been reading some books of late which deal with philosophical anthropology (what is man, from a philosophical perspective), with the issue of animal rights in the background. One of the more interesting and technical discussions is found in Evolution, Animal 'Rights,' and the Environment by James B. Reichmann, SJ. Reichmann, who wrote a more general Philosophy of the Human Person in the 80's, devotes this volume to a consideration of what makes human beings different from other creatures. In the process he treats the question of whether we differ from other animals in kind or in degree; that is, whether other animals share the same things we do, but only to a lesser degree, or whether we are essentially and different from other creatures.
In order to treat this issue in the depth he desires, Reichmann necessarily enters into an extended discussion of evolutionary theory. He does so because many of those who argue that we do not differ essentially from other animals base their case on the "fact" that we descended from other animals, and hence we logically cannot differ in essence from other creatures. (I put "fact" in quotes only because I am currently agnostic on the origins of our bodies.) Or rather, he demonstrates how strict Darwinistic evolutionary theory requires that one view humans and other animals as essentially the same, thus meaning that we humans possess no rights which can be denied to at least some other creatures.
Even more, Reichmann refers to the views of some ethicists and philosophers who argue that nonsentient and inanimate beings (i.e. plants and trees) may have rights! Taking up the work of Tom Regan, Reichmann states,
- After meticulously developing his case for animal rights, and grounding it on what he chooses to call "the inherent value of the animal," and initially limiting his argument to include only those animals that might be said to be subjects-of-a-life, Regan now grants the possibility that other living things, including animals who are not-subjects-of-a-life, and even some nonliving things, might truly possess rights.
Fr. Reichmann's text is a work devoted to demonstrating the intellectual case against such ridiculous arguments. If this is a topic which interests you, I'd highly recommend this book.