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majority of cases the electrical properties of living matter play no
special part in the life of the animal, they have become the specific
function of the eel's electric organs: an accident of nature has become
biologically significant.
One may suggest that the same sort of thing has happened with
mind. All the activities of the world-stuff are accompanied by
mental as well as by material happenings; in most cases, however,
the mental happenings are at such a low level of intensity that we cannot
detect them; we may perhaps call them "psychoid" happenings, to
emphasize their difference in intensity and quality from our own
psychical or mental activities. In those organs that we call brains,
however, the psychoid activities are, in some way, made to reinforce
each other until, as is clearly the case in higher animals, they reach a
high level of intensity; and they are the dominant and specific
function of the brain of man. Until we learn to detect psychoid
activities of low intensity, as we have learned to do with electrical
happenings, we cannot prove this. But already it has become the
simplest hypothesis that will fit the facts of developmental and
evolutionary continuity.
In evolution, science has not merely revealed the bridge that
provides continuity between man and lifeless matter, but has also
discovered what is perhaps the most important single biological fact
yet known—the fact of evolutionary progress. A great deal of evolu-
tion is mere diversification. New species constantly arise, adapted to
slightly different conditions, or produced by the biological accidents
of isolation or hybridization. Through this frill of diversity, how-
ever, there can be perceived a series of long-range trends, whose
course runs for millions or tens of millions of years. The great
majority of these trends are specializations. They fit the existing type
more closely to one mode of life, and in so doing cut it off from success
in others. In the evolution of higher mammals, for instance, one line
specialized as predators, and become the carnivores; another special-
ized in chewing and digesting foliage and herbage, and usually in
swift running, to become the ungulates; a third in flying—the bats;
a fourth in marine life—the whales and porpoises; and so on. It is
a universal rule that one-sided specializations eventually come to a
dead end. There is a point beyond which natural selection cannot
push them. It is impossible to be more perfectly streamlined than
a dolphin; when the horse stock had reduced its digits to one, it
could go no further; elephants are close to the limit of weight that
is possible for an efficient land animal. When a specialization has
reached its biomechanical limit, it remains unchanged—unless