divisions of the species for purposes of mutual protection are
thus met with even among the invertebrate animals.

Going now over to higher animals, we find far more instances
of undoubtedly conscious mutual help for all possible purposes,
though we must recognize at once that our knowledge even of the
life of higher animals still remains very imperfect. A large
number of facts have been accumulated by first-rate observers,
but there are whole divisions of the animal kingdom of which we
know almost nothing. Trustworthy information as regards fishes is
extremely scarce, partly owing to the difficulties of
observation, and partly because no proper attention has yet been
paid to the subject. As to the mammalia, Kessler already remarked
how little we know about their manners of life. Many of them are
nocturnal in their habits; others conceal themselves underground;
and those ruminants whose social life and migrations offer the
greatest interest do not let man approach their herds. It is
chiefly upon birds that we have the widest range of information,
and yet the social life of very many species remains but
imperfectly known. Still, we need not complain about the lack of
well-ascertained facts, as will be seen from the following.

I need not dwell upon the associations of male and female for
rearing their offspring, for providing it with food during their
first steps in life, or for hunting in common; though it may be
mentioned by the way that such associations are the rule even
with the least sociable carnivores and rapacious birds; and that
they derive a special interest from being the field upon which
tenderer feelings develop even amidst otherwise most cruel
animals. It may also be added that the rarity of associations