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 CHAPTER IX. PROOFS FROM VARIATION OF ORGANIC FORMS, ARTIFICIAL AND NATURAL. Limitation of the use of experiment in morphology—Unconscious experiments in breeding, and their results—Principles involved—Inheritance, immediate and ancestral—Effect of true breeding long continued—Method of selection illustrated by diagram—Formation of a race—Process the same in nature—Show selective effect of physical environment—Of organic environment—Of migrations—Of unlimited time—Other factors of change, and their effects shown in nature and in domestication—Differences between artificial and natural species 222 First difference, reversion—The tendency to reversion described—The reason explained—Illustrated by the case of the pointer 229 Second difference, intermediate forms—Reason is, these are eliminated in nature 232 Third difference, cross-fertility—Natural species are usually cross-sterile—Degrees of cross-sterility—Two bases of species, morphological and physiological—Two kinds of isolation, sexual repugnance and cross-sterility—Latter most essential—Illustrated by plants and hermaphrodite animals—Former only higher animals—Natural