XViii CONTENTS OF VOL. I. 



tion — Both are cases of one law — Instance of the effect of an accustomed sound 

 — The same is true of the unconscious life — Effect of medicines and stimulants 

 — Action of the heart under a stimulus— General law respecting passive im- 

 pressions — Instance of climbing plants— Organs grow with exercise — Lungs, 

 muscles, and brain — Laws of habit are true of both mind and bodj' — Laws of 

 habit do not account for every particular habit — Voluntary actions may become 

 habitual — This will not account for all habits — Question of the origin of species — 

 Summary — Habit — Hereditary transmission — Variation— Disuse— Prominence 

 — Tenacity — Reversion — Passive impressions — Effect of habit on organs — Great 

 changes are destructive— This is not always physically explicable — Organisms 

 are destroyed by changes that they cannot become habituated to — Great and 

 sudden changes of circumstances are destructive — CoiTespondLug changes in 

 habit are impossible — Great changes, if not sudden, are often not destructive— 

 Corresponding changes in habit are possible — Adaptation, how effected— Active 

 and passive habits — Change of climate — Change of food— Improvement in sight 

 and fleetness. 

 Note : Grmoth of Organs ivith Exercise .-- Why do organs grow with exercise 1 — 

 Herbert Spencer's theory — Woody iibre — Animal tissues — Possible nervous 

 action in mcreasiug nutrition in exercised parts — Increased flow of blood to 

 exercised parts, possibly due to relaxation of the nerves of the arteries. 



Pp. 167—186 



CHAPTER XVI. 



THE LAWS OF VARIATION. 



Changes of habit, functionally produced, and spontaneous — Benefit of slight 

 changes — Change of air — Change of seed — Benefit of slight mixtures of race — 

 Mixtures of unlike races — Slight changes are agreeable, great ones disagi-eeable 

 • — Summary — Effect of confinement to a small area — Sexuality — Generation is 

 only a modification of the general vital process— Reproduction of Algae — 

 Simplest form of sexual reproduction in unicellular Algaa — Its essential con- 

 dition is mixture of germinal matter from two sources — Zygnema : simplest form 

 of sexual distinction — Purpose of sexual distinction, to increase the difference 



• of the two sources — Hermaphrodite animals, and plants, not always self-fer- 

 tilizing — Agency of insects in fertilizing flowers — Sexuality distinct fi-om the 

 existence of separate generative organs : depends on the necessity of slight 

 changes — No species can live an indefinite time without sexual reproduction- 

 Variability promoted by slight change of conditions, and by mixture of races- 

 Variability and modifiability — Spontaneous variation, how far connected with 

 the laws of habit : originates only with new individuals — "Sporting" plants — 

 Variations are most abtmdant in cases of sexual generation — Intermediate 

 breeds are difficult to obtain — Reversion common in mixed breeds — Otter sheep 

 — Variation usually slow among animals : often sudden among plants — Poppy- 

 Ferns — Datura tatula — Only some races variable, and some characters of those 

 — Correlation of variations — Homologous parts vary together — Mental and 

 motor characters are more variable than fomiative ones — The minutest struc- 



■ ture is the least variable — Organs and tissues — Teeth — Shells— Muscular tissue 



