METHODS FOB PROVING ORIGIN OF SPECIES. 11 



tropics, have been once naturalized ; that is to say, they 

 have, from geographical and physical circumstances, 

 passed from one region to another. When, in 1855, I 

 put forward the idea that conditions anterior to our 

 epoch determined the greater number of the facts of the 

 actual distribution of plants — this was the sense of 

 several of the articles, and of the conclusion of my two 

 volumes of geographical botany ^ — it was received with 

 considerable surprise. It is true that general considera- 

 tions of palaeontology -had just led Dr. Unger,^ a German 

 savant, to adopt similar ideas, and before him Edward 

 Forbes had, with regard to some species of the southern 

 counties of the British Isles, suggested the hypothesis 

 of an ancient connection with Spain.* But* the proof 

 that it is impossible to explain the habitations of the 

 whole number of present species by means of the con- 

 ditions existing for some thousands of years, made a 

 greater impression, because it belonged more especially 

 to the department of botanists, and did not relate to 

 only a few plants of a single country. The hypothesis 

 suggested by Forbes became an assured fact an i capable 

 of general application, and is now a truism of science. AH 

 that is written on geographical or zoological botany rests 

 upon this basis, which is no longer contested. 



This principle, in its application to each country and 

 each species, presents a number of difficulties ; for when 

 a cause is once recognized, it is not always easy to dis- 

 cover how it has affected each particular case. Luckily, 

 so far as cultivated plants are concerned, the questions 

 which occur do not make it necessary to go back to 

 very ancient times, nor to dates which cannot be defined 

 by a given number of years or centuries. No doubt the 

 modern specific forms date from a period earlier than 

 the great extension of glaciers in the northern hemi- 



' Alph. de Candolle, O^ographie Botanigve Raisonnde, chap. x. p. 

 1055; chap, xi., xix., xxvii. 



^ Unger, Versuch einer Qeschichfe der Pfianisenwelf, 1852. 



' Forbes, On the Connection ietween the Distribution of the Existing 

 Fauna and Flora of the British Isles, with the Geological Changes which 

 have affected their Area, in 8vo, Memoirs of the Qeological Survey, vol. i. 

 1846. 



