E. MAYR 15 



was introduced into the biological literature by the English 

 naturalist John Ray and confirmed by the Swedish naturalist 

 Linnaeus. It is based not on difference but on distinction, and 

 this distinction in turn is characterized bv a definite mutual re- 

 lationship, namely that of reproductive isolation. The word 

 "species" is here best defined in combination with the word 

 "different." The relationship of two "different species" can be 

 objectively defined as reproductive isolation. We have, thus, an 

 objective yardstick for this species concept, something that is ab- 

 sent in all others. Philosophers have objected to the use of the 

 terms "objective" or "real" for species, and it may be more 

 neutral to use the terms arbitrary or nonarbitrary (Simpson, 

 1951). Presence or absence of interbreeding of two populations 

 in a nondimensional system is a completely nonarbitrary criterion. 

 Since the nondimensional species concept is based on a relation- 

 ship, the word species is here equivalent to words like, let us say, 

 the word brother, which also has a meaning only with respect 

 to a second phenomenon. An individual is a brother only with 

 respect to someone else. Being a brother is not an inherent prop- 

 erty as hardness is a property of a stone. Describing a presence 

 or absence relationship makes this species concept nonarbitrary. 



This species concept seems so self-evident to every naturalist 

 that it is only rarely put in words. That the species is more than 

 an aggregate of individuals, held together by a biological bond, 

 has long been realized, as was pointed out in the historical survey 

 above. The interbreeding within the species is more conspicuous, 

 and it was thus more often emphasized than is the reproductive 

 isolation against other species. Eimer, as early as 1889 (p. 16) 

 defined species as "groups of individuals which are so modified 

 that successful interbreeding [with other groups] is no longer 

 possible." The first author, however, who stated the nondimen- 

 sional species concept in its full extent and implication was 

 Jordan (1905). 



In spite of its theoretical superiority, the nondimensional 

 species has a number of serious drawbacks (which will be dis- 

 cussed later), particularly its limitation to sexually reproducing 

 species and to such without the dimensions of space and time. 



