166 The Origin of Species by Mutation. 



hope to obtain experimental proof of their common de- 

 scent : the theory of Descent as apphed to groups of these 

 units is, and will probably always remain, a comparative, 

 science. 



At the time when the Linnean view that species had 

 been separately created was generally accepted, it was 

 naturally a very important matter to decide which forms 

 should be regarded as species. I have already endeavored 

 to give some account of the broad features of the con- 

 troversy which raged round this question during the 

 period just before Darwin's work appeared.^ 



Since the hypothesis of special creation of species 

 was given up, the view that the Linnean species really were 

 the units of the system was fostered by the persistence 

 of binary nomenclature. But w^e are liable to forget 

 that these species do not correspond to the units which 

 exist in nature, but to groups of them. This is a fact 

 which is clearly recognized and repeatedly asserted by 

 the best systematists.^ Linnaeus himself, as we have 

 seen, regarded his species as groups^ and not as simple 

 things, and De Candolle often speaks of them as col- 

 lective. 



The classification of plants into groups called species 

 has exactly the same value and meaning as their classi- 

 fication under the headings of genera, families and so 

 forth. So long as our knowledge as to what are the 

 real units of the system is as incomplete as it is at pres- 

 ent, systematists and students of distribution, no less 

 than evolutionists will have to be content to deal with 



^ See chapter II, pp. 16-28. 



^Alph. De Candolle, La Phytographie', and De VOrigine des 

 Especes cultivees, 1883, p. 372. 



^ A good example of this is afforded by the species Homo sapiens. 



