Chances against concordant variations.--Examples of discordant ones.--Concordant variations not unlikely on a non-Darwinian evolutionary hypothesis.--Placental and implacental mammals.--Birds and reptiles.--Independent origins of similar sense organs.--The ear.--The eye.--Other coincidences.--Causes besides Natural Selection produce concordant variations in certain geographical regions.--Causes besides Natural Selection produce concordant variations in certain zoological and botanical groups.--There are homologous parts not genetically related.--Harmony in respect of the organic and inorganic worlds.--Summary and conclusion ... _Page_ 63 CHAPTER IV. _MINUTE AND GRADUAL MODIFICATIONS._ There are difficulties as to minute modifications, even if not fortuitous.--Examples of sudden and considerable modifications of different kinds.--Professor Owen's view.--Mr. Wallace.--Professor Huxley.--Objections to sudden changes.--Labyrinthodont.--Potto.--Cetacea.--As to origin of bird's wing.--Tendrils of climbing plants.--Animals once supposed to be connecting links.--Early specialization of structure.--Macrauchenia.--Glyptodon.--Sabre-toothed tiger.--Conclusion ... _Page_ 97 {ix} CHAPTER V. _AS TO SPECIFIC STABILITY._ What is meant by the phrase "specific stability;" such stability to be expected _a priori_, or else considerable changes at once.--Rapidly increasing difficulty of intensifying race characters; alleged causes of this phenomenon; probably an internal cause co-operates.--A certain definiteness in variations.--Mr. Darwin admits the principle of specific stability in certain cases of unequal variability.--The goose.--The peacock.--The guinea fowl.--Exceptional causes of variation under domestication.--Alleged tendency to reversion.--Instances.--Sterility of hybrids.--Prepotency of pollen of same species, but of different race.--Mortality in young gallinaceous hybrids.--A bar to intermixture exists somewhere.--Guinea-pigs.--Summary and conclusion ... _Page_ 113 CHAPTER VI. _SPECIES AND TIME._ Two relations of species to time.--No evidence of past existence of minutely intermediate forms when such might be expected _a priori_.--Bats, Pterodactyles, Dinosauria, and Birds.--Ichthyosauria, Chelonia, and Anoura.--Horse ancestry.--Labyrinthodonts and Trilobites.--Two subdivisions of the second relation of species to time.--Sir William Thomson's views.--Probable period required for ultimate specific evolution from primitive ancestral forms.---Geometrical increase of time required for rapidly multiplying increase of structural differences.--Proboscis monkey.--Time required for deposition of strata necessary for Darwinian evolution.--High organization of Silurian forms of life.--Absence of fossils in oldest rocks.--Summary and conclusion ... _Page_ 128 CHAPTER VII. _SPECIES AND SPACE._