n spite of
several ingenious hypotheses that have been advanced to explain it.

Fatigue, rest and sleep.

That fatigue, primarily an organic state, gives rise to fatigue
sensations and to a neural adjustment for rest--a disinclination to
work any longer--and that drowsiness is a somewhat different organic
state that gives an inclination to sleep--all this has been
sufficiently set forth in earlier chapters. Going to sleep is a
definite act, an instinctive response to the drowsy state. In the way
of preparatory reactions, we find many interesting performances in
birds and mammals, such as the curling up of the dog or cat to sleep,
the roosting of hens, the standing on one leg of some birds; and we
see characteristic positions adopted by human beings, but do not know
how far these are instinctive and how far acquired. Closing the eyes
is undoubtedly a native preparatory reaction for sleep.

Like the other responses to organic needs, rest and sleep figure
pretty largely in the behavior of the adult, as in finding or
providing a good place to sleep. Certainly if fatigue and sleep could
be eliminated, as some over-enthusiastic workers have pretended to
hope, life would be radically changed.

Instinctive Responses to Other Persons

We are next to look for action and emotion aroused by persons,
specifically--not by persons and things alike. Fear can be aroused by
persons, but also by things. In a social animal, such as man, almost
any instinct comes to have {146} social bearings. Eating and drinking
become social matters, and all the organic instincts figure in the
placing and making of a home. Home is a place of shelter against heat
and cold, it is a refuge from danger, it is where you eat and where
you sleep. It meets all these organic needs but--it is specially where
"your people" are.

Home is a place where _unlike_ persons foregather, male with female,
adults with children, and thus it symbolizes the "family instincts",
mating and child-care, which are responses to persons unlike in sex or
age. But home also illustrates very well the herd instinct, which is a
response to like persons, "birds of a feather flocking together". It
is not the single home that illustrates this, but the almost universal
grouping of homes into villages or cities.

The herd instinct or gregarious instinct.

It might be argued that a city or village was the result of economic
causes, or, in the olden days, a means of protection agai