CHAPTER VIII.. Proofs From Geographical Distribution of Organisms.

PROOFS FROM GEOGRAPHICAL DISTRIBUTION OF ORGANISMS.

Geographical faunas and floras--Conditions which limit 183

Temperature-regions--Illustrated by plants--In latitude and in elevation--Same in animal species 184

More perfect definition of regions--Range of different Taxonomic groups--Gradual shadings on borders of range-- Shadings out of individuals in number and vigor, but not in specific character--As if centers of origin--Effect of east and west barriers--Temperature regions repeated south of the equator, but not species--As if centers of origin 186

Continental faunas and floras--Temperature zones continuous, but not species--Reason: ocean barriers--As if centers of origin--Polar regions: one. Why--Temperate zone-- Different species in different continents--Species of United States and of Europe almost wholly different--As if origin local--Exceptions--(1) Introduced species--(2) Hardy or else wide-migrating species--(3) Alpine species--Tropical zone of two continents still more different--Same true of south temperate zone 188

Subdivisions of continental faunas and floras--Illustrated by fauna and flora of United States 191

Special Cases--Australia--Madagascar--Galapagos--River mussels 192

Marine species--Same principles applicable--Therefore organic forms grouped in regions, sub-regions, provinces, etc.-- Primary regions according to Wallace--According to Allen 192

Theory of the origin of geographical diversity--Specific centers of creation--Objections to. The element of time left out--Progressive change in unlimited time, or evolution the only rational explanation--This connects with geographical changes in geological times, especially the Glacial epoch-- Geographical diversity in other times 193

Most probable view of the general process--Last great period of change was the Glacial epoch--This, therefore, is the key to geographical distribution--Condition of things during the Glacial epoch---In America--Changes in temperature--In physical geography and in species--In Europe--Application of principles 196

(1) Australia--Characteristics of its fauna--Explanation of--Isolation very early--Position of marsupials and monotremes in the Taxonomic scale--Australia isolated before the Tertiary--Effect of competition on evolution 200

(2) Africa--African region defined--Two groups of its mammals, indigenes, and invaders--Effect of the invasion 204

(3) Madagascar--Characteristics of its fauna--Relation to African indigenes--Separated before the invasion-- Significance of its lemurs 205

(4) Island life--Two kinds of islands--Defined and illustrated by examples--(a) Continental islands-- General character of fauna--Illustrated by Madagascar, New Zealand, British Islands, coast-islands of California --Characteristics of the faunas of these explained--(b) Oceanic Islands--Defined--Characteristics of faunas and their origin 207

(5) Alpine species--Characteristics of and their origin explained--Migrations of Arctic species during Glacial times, and their isolation on mountains 215

Objection--Mode of change of species on borders of ranges-- Examples--Sweet-gum--Sequoia 217

Answer--Distribution of these forms in time, and their migrations--They are remnants--Intermediate forms are extinct 219