8 CONCEPTS AND DEFINITIONS 



long period I look at as the essence of a species" (letter to 

 Hooker, Oct. 22, 1864). Such constancy in time was the strongest 



gument in favor of a morphological species concept, but it 

 could be proved only by the comparison of individuals of differ- 

 ent generations. Different morphological "types" that are no more 

 different than mother and daughter or father and son can safely 

 be considered as conspecific. They are "of the same blood." It is 

 obvious that this early stress of descent was essentially the 

 consequence of a morphological species concept. Yet this con- 

 sideration of descent eventually led to a genetic species definition. 



Virtually all early species definitions regarded species only 

 as aggregates of individuals, unconnected except by descent, as 

 is evident not only from the writings of Robinet, Buffon, and 

 Lamarck, but also of much more recent authors (e.g., Britton, 

 1908; Bessey, 1908). The realization that these individuals are 

 held together by a supraindividualistic bond, that they form 

 populations, came only slowly. Illiger ( 1800 ) spoke of species 

 as a community of individuals which produce fertile offspring. 

 Brauer (1885) spoke of the "natural tie of blood relationship" 

 through which the "individuals of a species are held together," 

 and which "is not a creation of the human mind ... if species 

 were not objective, it would be incomprehensible that even the 

 most similar species mix only exceptionally and the more distant 

 species never." Plate (1914) was apparently the first to state ex- 

 plicitly the nature of this bond: "The members of a species arc 

 tied together by the fact that they recognize each other as be- 

 longing together and reproduce only with each other. The sys- 

 tematic category of the species is therefore entirely independent 

 of the existence of Man." Finally, in the language of current 

 population genetics this community becomes the "co-adapted 

 gene pool," again stressing the integration of the members of 

 the population rather than the aggregation of individuals (a 

 viewpoint which is of course valid only for sexually reproducing 

 organisms). 



The growth of thinking in terms of populations went hand in 

 hand with a growing realization that species were less a matter 

 of difference than of distinctness. "Species" in its earlier typolog- 



