GEOGRAPHICAL DISTRIBUTION OF ORGANISMS. 485 



Theories of the Origin of the Distribution of 

 Organisms.— We have given in outline the facts. The 

 question is, " How came it so ? " Before the advent of 

 the theory of evolution the most rational theory was 

 based on the ideas of permanence of specific types and centers 

 of specific origin, as if each species was made in its pres- 

 ent form and its present place, and thence spread as far 



Fig. 336. 



as it could without changing its character. It did not 

 deny some variation, but always within certain limits. 

 If the centre of a circle represents a specific type, then 

 radii will represent the variation in all directions, and 

 the circumference the limit of variations. Some species 

 are more and some less variable; the circle may be 

 larger or smaller, but in all there is a limiting line be- 

 yond which it is impossible to go without destroying the 



