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forces it farther along in the same direction, while constantly deepen-
ing the groove and so making it ever more impossible for the stock to
escape out of it into some other way of life. The dead end comes
when the specialization is so near its maximum possible perfection
that selection cannot force the stock any further.
A third and still rarer type of change is evolutionary progress, which
escapes the dead end awaiting specialization. It docs so because its
essence is all-round improvement, as opposed to the one-sided im-
provement that characterizes all specialization. It raises the general
level of life's performance, instead of merely improving performance
in respect of one particular mode of existence. The development of
a head and brain or of a blood-system were early steps in progressive
evolution, while the acquisition of u warm blood" arid so of a constant
internal temperature, or the gradual development in mammals of
higher mental faculties such as association and the capacity for
learning by experience, are later examples.
The net result of evolutionary progress can be defined as the raising
of the upper level attained by life in respect of certain very general
properties—greater control; greater independence; greater harmony
of construction; greater capacity for knowledge (and, we may prob-
ably add, for emotion). More concretely, it has permitted the rise
of a succession of what the biologist calls dominant groups, because
they spread and evolve rapidly, cause the extinction of many repre-
sentatives of other groups, and play a new and predominant role on
the evolutionary stage. The last three dominant groups in life's
history have been the reptiles, the mammals, and man, each later
one arising from an unspecialized branch of the one before. Most
(or, in some cases, all) the branches of a dominant group undergo
specialization, and then eventually come to a dead end, cither by
ceasing to evolve, or by the still deader end of complete extinction,
as with most of the reptilian specializations, like the Dinosaurs,
Ichthyosaurs, and Pterodactyls.
I said that progressive lines were rare. If we define progress strictly
as capacity for unlimited further avoidance of dead ends, there has
only been one progressive line in the whole of evolution—that which
has led in its later stages through fish, amphibian, reptile, and mam-
mal to man; for it appears established that all other lines have come
to an evolutionary dead end well before the later part of the tertiary
Thus, in the broad view, evolution as a process consists of one line
of unlimited progress among thousands of long-range trends toward
specialization, each of these latter in turn beset with a frill, so to