CHAPTER IV 



EVOLUTION THE KEY TO DISTRIBUTION 



Importance of the Doctrine of Evolution — The Origin of New Species — 

 Variation in Animals— The Amount of Variation in North American 

 Birds — How New Species arise from a Variable Species — Definition and 

 Origin of Genera — Cause of the Extinction of Species — The Rise and 

 Decay of Species and Genera — Discontinuous Specific Areas, why Rare — 

 Discontinuity of the Area of Parus Palustris — Discontinuity of Emberiza 

 Schoeniclus — The European and Japanese Jays — Supposed Examples of 

 Discontinuity among North American Birds — Distribution and Antiquity 

 of Families — Discontinuity a proof of Antiquity — Concluding Remarks. 



In the preceding chapters we have explained the general 

 nature of the jijhenomena presented by the distribution of 

 animals, and have illustrated and defined the new 

 geographical division of the earth which is found best to 

 a^ree with them. Before we s^o further into the details of 

 our subject, and especially before we attempt to trace the 

 causes which have brought about the existing biological 

 relations of the islands of the globe, it is absolutely 

 necessary to have a clear comprehension of the collateral 

 facts and general principles to which we shall most 

 frequently have occasion to refer. These may be briefly 

 defined as, the powers of dispersal of animals and plants 

 under different conditions, such as geological and climatal 

 changes, and the origin and development of species and 

 groups by natural selection. This last is of the most 

 fundamental importance, and its bearing on the dispersal 



