es 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._

The geographical distribution of animals presents difficulties.--These not
insurmountable in themselves; harmonize with other
difficulties.--Fresh-water fishes.--Forms common to Africa and India; to
Africa and South America; to China and Australia; to North America and {x}
China; to New Zealand and South America; to South America and Tasmania; to
South America and Australia.--Pleurodont lizards.--Insectivorous
mammals.--Similarity of European and South American frogs.--Analogy between
European salmon and fishes of New Zealand, &c.--An ancient Antarctic
continent probable.--Other modes of accounting for facts of
distribution.--Independent origin of closely similar forms.--Conclusion ...
_Page_ 144

CHAPTER VIII.

_HOMOLOGIES._

Animals made up of parts mutually related in various ways.--What homology
is.--Its various kinds.--Serial homology.--Lateral homology.--Vertical
homology.--Mr. Herbert Spencer's explanations.--An internal power
necessary, as