and lowly organised structures variable -- Parts developed in an unusual manner are highly variable; specific characters more variable than generic; secondary sexual characters variable -- Species of the same genus vary in an analogous manner -- Reversions to long-lost characters -- Summary.
CHAPTER VI.
DIFFICULTIES OF THE THEORY.
Difficulties of the theory of descent with modification -- Absence or rarity of transitional varieties -- Transitions in habits of life -- Diversified habits in the same species -- Species with habits widely different from those of their allies -- Organs of extreme perfection -- Modes of transition -- Cases of difficulty -- Natura non facit saltum -- Organs of small importance -- Organs not in all cases absolutely perfect -- The law of Unity of Type and of the Conditions of Existence embraced by the theory of Natural Selection.
CHAPTER VII.
MISCELLANEOUS OBJECTIONS TO THE THEORY OF NATURAL SELECTION.
Longevity -- Modifications not necessarily simultaneous -- Modifications apparently of no direct service -- Progressive development -- Characters of small functional importance, the most constant -- Supposed incompetence of natural selection to account for the incipient stages of useful structures -- Causes which interfere with the acquisition through natural selection of useful structures -- Gradations of structure with changed functions -- Widely different organs in members of the same class, developed from one and the same source -- Reasons for disbelieving in great and abrupt modifications.
CHAPTER VIII.
INSTINCT.
Instincts comparable with habits, but different in their origin -- Instincts graduated -- Aphides and ants -- Instincts variable -- Domestic instincts, their origin -- Natural instincts of the cuckoo, molothrus, ostrich, and parasitic bees -- Slave-making ants -- Hive-bee, its cell- making instinct -- Changes of instinct and structure not necessarily simultaneous -- Difficulties on the theory of the Natural Selection of instincts -- Neuter or sterile insects -- Summary.
CHAPTER IX.
HYBRIDISM.
Distinction between the sterility of first crosses and of hybrids -- Sterility various in degree, not universal, affected by close interbreeding, removed by domestication -- Laws governing the sterility of hybrids -- Sterility not a special endowment, but incidental on other differences, not accumulated by natural selection -- Causes of the sterility of first crosses and of hybrids -- Parallelism between the effects of changed conditions of life and of crossing -- Dimorphism and Trimorphism -- Fertility of varieties when crossed and of their mongrel offspring not universal -- Hybrids and mongrels compared independently of their fertility -- Summary.
CHAPTER X.
ON THE IMPERFECTION OF THE GEOLOGICAL RECORD.
On the absence of intermediate varieties at the present day -- On the nature of extinct intermediate varieties; on their number -- On the lapse of time, as inferred from the rate of denudation and of deposition -- On the lapse of time as estimated in years -- On the poorness of our palaeontological collections -- On the intermittence of geological formations -- On the denudation of granitic areas -- On the absence of intermediate varieties in any one formation -- On the sudden appearance of groups of species -- On their sudden appearance in the lowest known fossiliferous strata -- Antiquity of the habitable earth.
CHAPTER XI.
ON THE GEOLOGICAL SUCCESSION OF ORGANIC BEINGS.
On the slow and successive appearance of new species -- On their different rates of change -- Species once lost do not reappear -- Groups of species follow the same general rules in their appearance and disappearance as do single species -- On extinction -- On simultaneous changes in the forms of life throughout the world -- On the affinities of extinct species to each other and to living species -- On the state of development of ancient forms -- On the succession of the same types within the same areas -- Summary of preceding and present chapter.
CHAPTER XII.
GEOGRAPHICAL DISTRIBUTION.
Present distribution cannot be accounted for by differences in physical conditions -- Importance of barriers -- Affinity of the productions of the same continent -- Centres of creation -- Means of dispersal by changes of climate and of the level of the land, and by occasional means -- Dispersal during the Glacial period -- Alternate Glacial periods in the north and south.
CHAPTER XIII.
GEOGRAPHICAL DISTRIBUTION -- CONTINUED.
Distribution of fresh-water productions -- On the inhabitants of oceanic islands -- Absence of Batrachians and of terrestrial Mammals -- On the relation of the inhabitants of islands to those of the nearest mainland -- On colonisation from the nearest source with subsequent modification -- Summary of the last and present chapter.
CHAPTER XIV.
MUTUAL AFFINITIES OF ORGANIC BEINGS: MORPHOLOGY -- EMBRYOLOGY -- RUDIMENTARY ORGANS.
Classification, groups subordinate to groups -- Natural system -- Rules and difficulties in classification, explained on the theory of descent with modification -- Classification of varieties -- Descent always used in classification -- Analogical or adaptive characters -- Affinities, general, complex and radiating -- Extinction separates and defines groups -- Morphology, between members of the same class,
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