THEORY OF MIGRATIONS. 113 



scientific investigation of these conditions, to the Science of 

 Distribution, or Chorology ; but until Darwin, people were 

 satisfied to observe the phenomena of Chorology, and tried 

 principally to establish the demarcations of the distribu- 

 tions of existing organic groups of greater or less extent 

 But the causes of the remarkable phenomena of distribu- 

 tion, the reasons why some groups exist only here, others 

 only there, and why there are such numerous divisions ol 

 the various species of plants and animals, it was impossible 

 to explain. The Doctrine of Descent, for the first time, fur- 

 nishes the key to the solution of this problem also ; it alone 

 puts us in the right way to obtain an explanation, by ^how 

 ing us that the various species and groups of species spring 

 from common ancestral species, the widely diverging pos- 

 terity of which gradually spread over the whole earth. 

 Yet for every group of species there must be assumed a so- 

 called " centre of creation " — that is, a common cradle, or 

 original habitat, in which the common ancestral species of 

 a group first evolved, and from which their immediate 

 descendants dispersed in different directions. Individuals 

 of these migrated species became in their turn the ances- 

 tral species of new groups, which again, by active and 

 passive migration, dispersed; and so on. As every form 

 after its migration adapted itself to new conditions of 

 existence in its new home, it underwent modification, and 

 gave rise to new series of forms. 



Darwin, by the Theory of Descent, was the first to 

 establish this highly important doctrine of active and 

 passive migrations. At the same time he correctly pointed 

 out the significance of the important chorological relations 

 between the living population of each region and their fossil 



