The Growth of Biological Thought: Diversity, Evolution, and Inheritance
The Possible and the Actual
To write a history of the world, a man would have to be slightly crazy. He would have to be possessed by some single idea which, he believed, could illuminate the whole course of history, and in the light of which all events could be judged. To write a history of biological thought, particularly one confined to thoughts about biological diversity rather than physiological mechanism, is a less ambitious undertaking, and Ernst Mayr is certainly not crazy. But the task is sufficiently formidable, and Mayr has succeeded at it only because he does indeed have a single, unified vision, which he uses to evaluate scholars from Aristotle to Theodosius Dobzhansky and R.A. Fisher.
His vision is that the history of biology has been a struggle between two world views—essentialism and population thinking. The essentialist view holds that there are a finite number of kinds of animals and plants, each kind characterized by certain essential features which it is the business of the biologist to recognize. These “kinds” correspond to what we today call species. The members of a species share an identical essence. They may differ from one another in various ways, as people differ in height, hair color, and fingerprints, but these differences are accidental and unimportant; they do not alter the essence. The populational view holds that individuals are bound together, not by the possession of a common essence, but by the fact that they interbreed.
The supreme essentialist was Linnaeus. His attitude derived in part from certain features of the real world (which I will describe in a moment) and in part from the philosophy of Aquinas. Linnaeus’s great French contemporary, Buffon, held views that derived from the alternative medieval philosophy of nominalism. He held that individuals constitute the only reality. We classify them altogether arbitrarily in groups to which we give names (hence “nominalism”). By the act of naming species, we create them.
It is interesting that, starting from such diametrically opposed positions, Linnaeus and Buffon were brought closer together as their knowledge of actual diversity increased. Linnaeus never abandoned the concept of essences, but he did abandon his earlier claim that “we count as many species as were created in the beginning.” In its place he put the genus—oddly, because if any category is real it is the species—and suggested that species might have arisen by hybridization. He was driven to this position by the sheer impossibility of characterizing each species by fixed and unvarying essences. Buffon never accepted essences, but he was driven to recognize that the groups into which organisms fall are not arbitrary and man-made. In other words, he admitted that real divisons exist independently of our judgments.
An interesting confirmation of the reality of species, which has only recently been appreciated, is that primitive peoples usually recognize the same species as do modern taxonomists. The process that ensures the uniformity of the members of a species in any one place is that they interbreed; different species …