CHAPTER IV

EVOLUTION AS 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

_Pages_ 55-71