E. MAYR 7 



the reproductive isolation of populations. Let us try to trace the 

 emergence of these and related concepts. 



It is not surprising that species were considered merely "cate- 

 gories of thought" by many writers in periods so strongly 

 dominated by idealistic philosophy as were the eighteenth and 

 nineteenth centuries. Thoughts as that expressed in the above 

 quoted statement of Robinet were echoed by Agassiz, Mivart, and 

 particularly among those paleontologists who considered their 

 task merely the classification of "objects" ( = fossil specimens ) . 

 In opposition to this, an increasingly strong school developed 

 which considered species as "definable," "objective," "real." Lin- 

 naeus was, of course, the original standard bearer of this school 

 to which also belonged Cuvier, De Candolle, and many taxon- 

 omists in the first half of the nineteenth century. They supported 

 their case sometimes by purely morphological arguments such 

 as Godron ( 1853 ) who stated : "c est un fait incontestable que 

 toutes les especes animales et vegetables se separent les unes des 

 autres par de caracteres absolues et tranchees." Others used a 

 more biological argument, as I will discuss below. 



What is unexpected for this pre-Darwinian period, however, 

 is the frequency with which "common descent" is included in 

 species definitions. When such an emphatically anti-evolutionary 

 author as v. Baer (1828) defines the species as "the sum of the 

 individuals that are united by common descent," it becomes 

 evident that he does not refer to evolution. What is really meant 

 is more apparent from Ray's species definition (1686) or a state- 

 ment by the Swedish botanist Oeder ( 1764 ) that it characterizes 

 species "dass sie aus ihres gleichen entsprungen seien und wieder 

 ihres gleichen erzeugen." Expressions like "community of origin" 

 or "individus descendants des parents communs" (Cuvier) are 

 frequent in the literature. These are actually attempts at recon- 

 ciling a typological species concept ( with its stress of constancy ) 

 with the observed morphological variation. Constancy was a 

 property of species taken very seriously not only by Linnaeus 

 and his followers but curiously enough also by Lamarck and by 

 Darwin himself: "The power of remaining constant for a good 



