CHAP. I.] INTRODUCTION. 7 



type." He likewise believed in specific centres of 

 distribution. He held that all the individuals com- 

 posing a species had descended from a single pro- 

 genitor, or from two, according as the sexes might be 

 united or distinct, and that consequently the idea of a 

 species involved the idea of the relationship in all the 

 individuals of common descent; and the converse, that 

 there could by no possibility be community of descent 

 except in living beings which possessed the same 

 specific characters. He supposed that the original 

 individual or pair was created at a particular spot 

 where the conditions were suitable for its existence 

 and propagation, and that the species extended and 

 migrated from that spot on all sides over an area of 

 greater or less extent, until it met with some natural 

 barrier in the shape of unsuitable conditions. No 

 specific form could have more than a single centre of 

 distribution. If its area appeared to be broken up, a 

 patch not in connection with the original centre of 

 distribution occurring in some distant locality, it was 

 accounted for by the formation, through some geolo- 

 gical change after the first spread of the species, of a 

 barrier which cut off a part of its area; or to some 

 accidental transport to a place where tbe conditions 

 were suflEiciently similar to those of its natural original 

 habitat to enable it to become naturalized. No species 

 once exterminated was ever recreated, so that in those 

 few cases in which we find a species abundant at one 

 period over an area, absent over the same area for a 

 time, and recurring at a later period, it must be ac- 

 counted for by a change in the conditions of the area 

 which forced the emigration of the species, and a sub- 

 sequent further change which permitted its return. 



