Extreme variation in the meaning of the term “species” throughout the history of biology has often frustrated attempts of historians, philosophers and biologists to communicate with one another about the transition in biological thinking from the static species concept to the modern notion of evolving species. The most important change which has underlain all the other fluctuations in the meaning of the word “species” is the change from it denoting such metaphysical entities as essences, Forms or Natures to denoting classes of individual organisms. Several authors have taken notice of the role of metaphysics in the work of particular biologists. An attempt will be made in this paper to present a systematic investigation of the role which metaphysics has played in the work of representative biologists throughout the history of biology, especially as it relates to their species concepts.
* I wish to thank Ernst Mayr for criticizing an early draught of this paper and Yukio Kachi for his assistance in checking the translations of certain Greek passages. Early versions of this paper were read before the Philosophy Department of the University of Texas and the palaeozoologists at the University of Chicago. Their comments and criticisms were also much appreciated.