{Classes, styles, conflicts: The biological realm of \rlap{\LaTeX}} {Didier Verna} {Every \LaTeX\ user faces the ``compatibility nightmare'' one day. With so much intercession capabilities at hand (\LaTeX\ code being able to redefine itself at will), a time comes inevitably when the compilation of a document fails, due to a class/style conflict. In an ideal world, class/style conflicts should only be a concern for package maintainers, not end-users of \LaTeX. Unfortunately, the world is real, not ideal, and end-user document compilation does break. As both a class/style maintainer and a document author, I tried several times to come up with some general principles or a systematic approach to handling class/style cross-compatibility in a smooth and gentle manner, but I ultimately failed. Instead, one Monday morning, I woke up with this vision of the \LaTeX\ biotope, an emergent phenomenon whose global behavior cannot be comprehended, because it is in fact the result of a myriad of ``macro''\hyph interactions between small entities, themselves in perpetual evolution. In this presentation, I would like to draw bridges between \LaTeX\ and biology, by viewing documents, classes and styles as living beings constantly mutating their gene\TeX\ code in order to survive \CS{renewcommand} attacks. }