68 'WHAT IS A SPECIES?' 



and the whole edition of 1,250 copies sold on the day 

 of issue. On November 29 he wrote to Asa Gray: — 

 * You speak of species not liaving- an)- material base 

 to rest on, but is this an)- L;reater hardship than deciding 

 what deserves to be called a variety, and be designated 

 by a Greek letter ? When I was at systematic work 

 I know I longed to have no other difficulty (great enough) 

 than deciding whether the form was distinct enough 

 to deserve a name, and not to be haunted with undefined 

 and unanswerable questions whether it was a true species. 

 What a jump it is from a well-marked variety, produced 

 by natural cause, to a species produced by the separate 

 act of the hand of God ! But I am running on foolishly. 

 By the way, I met the other day Phillips, the palaeonto- 

 logist, and he asked me, " How do you define a species ? " 

 I answered, " I cannot." Whereupon he said, " At last 

 I have found out the only true definition, — any form 

 which has ever had a specific name ! 



») » 



The idea of Sy)iga77iy underlies tJie gradual iransitio)is 

 as 7>.'ell as the more iniiforvi resemblances charac- 

 teristic of Diagnostic Species. 



The idea of a species as an interbreeding community, 

 as syngamic, is, I believe, the more or less acknowledged 

 foundation of the importance given to transition. This 

 will become clearer from the consideration of a concrete 

 example. The common black-and-white Danaine butter- 

 fly, Ama2iris niavius of West Africa, is represented on 

 the East and South-East Coasts by a very similar butter- 

 fl)-, distinguished by the greater size of the largest white 

 patch, and of the white spot in the cell of the fore-wing. 

 Both forms are very constant in the areas over which they 

 were known, and on these constant easil)- recognizable 

 characters the eastern butterfly was described as a dis- 

 tinct species under the name of A. doniinicanus. Auri- 

 villius, however, in his valuable Catalogue refuses to 

 recognize this latter as a distinct species, and considers it 

 as the domi7iicanus variety of niavius. Through the 



* More Letters, vol. i, p. 127, Letter 79. 



