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 the conditions 
were sufficiently 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. 
