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 phenomena presented by the distribution of animals, and 
have illustrated and defined the new geographical division of the 
earth which is found best to agree with them. Before we go 
further into the details of our subject, and especially before we 
attempt to trace the causes which have brought about the exist- 
ing 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 — 
geological and climatal changes — and the origin and develop- 
ment of species and groups by natural selection. This last is of 
the most fundamental importance, and its bearing on the dispersal 
of animals has been much neglected. We therefore devote the 
present chapter to its consideration. 
As we have already shown in our first chapter that the distri- 
bution of species, of genera, and of families, present almost 
