Ray defined a species as all similar individuals exhibit- 

 ing constant characters from generation to generation, 

 while, according to Linnseus, each species was descended 

 from an originally created pair. "There are as many 

 species", he said, "as issued in pairs from the Creator's 

 hands," and each species expressed an idea in the Divine 

 mind. This conception of species was generally held un- 

 til the pubHcation of the "Origin of Species" in 1859. 



According to Darwin, however, classification meant 

 the grouping of organisms to show relationships due to 

 community of descent. He likened the affinities of all the 

 beings of the same class to a great tree where the green and 

 budding twigs represent existing species and those pro- 

 duced during former years the long succession of extinct 

 species. 



As a result, the Linnaean conception of species has been 

 modified. Darwin and many investigators since his time 

 have shown that Linnaean species are really groups of close- 

 ly related forms, and that it is often impossible in nature 

 to determine their limits on account of the large number of 

 intermediate forms. 



The idea of species is, therefore, an "abstract one: in 

 nature there are no species — only individuals." The 

 term, however, is still retained as a convenient one by sys- 

 tematists to designate all the individuals which agree in all 

 essential features and live, as nearly as possible, under 

 similar external conditions. 



De Vries believes that his "elementary species" and 

 "varieties" are the real units in nature. Individuals are 

 considered as aggregates of "unit characters" which may 

 combine in various ways, thus making it impossible to keep 

 the group called a "species" as a definite entity. 



The classification of organisms has, therefore, passed 

 through four eras: 1. the Pre- Linnaean, 2. the Pre-Darwin- 

 ian, 3. the Pre-Mendelian, and 4. the Post-Mendelian. 



The Pre-Linnaean era ( — 1753) is characterized by 

 crude unsystematic attempts at classification; the Pre- 

 Darwinian era (1753-1859) by the introduction of system, 

 frequently unnatural, and with distinct barriers between 

 the members of each group; the Pre-Mendelian era (1859- 

 1900) by the evolutionary conception of community of 

 descent with the barriers between species more or less 

 broken down; and the Post-Mendelian era (1900- ) by 

 the unit-character conception of individuals and the in- 

 troduction of De Vries' 'elementary species' and 'varieties' 

 as the real units in nature. 



