206 NATURAL HISTORY. 



world. Forbes therefore, like Cuvier before him, believed 

 that each 'species' of animals and plants was so far 

 permanent, that though it might be exterminated alto- 

 gether, and thus become ' extinct ' it could not become 

 changed into a new species. This implies the further 

 belief that 'variation' is strictly limited and definite in 

 amount, and that ' varieties ' of animals are mere temporary 

 modifications instead of being 'incipient species.' To use 

 Forbes's own words, 'every true species presents in its 

 individuals, certain features, specific characters, which dis- 

 tinguish it from every other species ; as if the Creator had 

 set an exclusive mark or seal on each type.' He also 

 believed that what we call a ' genus,' that is to say, a group 

 of allied species, is similarly permanent. He believed 

 to use his own expression that ' a genus is an abstraction, 

 a divine idea .... a true genus is natural, and, as 

 such, is not dependent on man's will.' 



This belief in the fixity of species carries with it, as an 

 almost necessary corollary, a belief in what are known as 

 'specific centres.' On the view of the permanence of 

 species, each particular species must have come into 

 existence at a particular moment of time and at a 

 particular place in space. That place must have been the 

 point where the first progenitor, or pair of progenitors, of 

 the species was created. This place must be supposed to 

 be one specially adapted for the life of the species, and it 

 constitutes the 'specific centre' for the species. From 

 this centre the species would gradually diffuse itself by 

 migration, over a more or less extensive area, till its 

 further progress would be stopped by meeting conditions 

 unsuitable for its existence. Hence, each species at the 



