ABSTRACT
I CONSIDER it a very great honour for me to speak on the occasion of the fiftieth anniversary of the death of Pasteur about the very work with which he began his scientific career. It was indeed more than this, for, as I hope to show, this question of molecular asymmetry not only furnishes the key to all Pasteur's subsequent work, but also marks an important turning-point of the science of the nineteenth and twentieth centuries.
