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by the Pleistocene period, or even earlier, only one was left. Let us
remember that we can and mxist judge early progress in the light of its
latest steps. The most recent step has been the acquisition of con-
ceptual thought, which has enabled man to dethrone the non-human
mammals from their previous position of dominance. It is biologic-
ally obvious that conceptual thought, could never have arisen save in
an animal, so that all plants, both green and otherwise, are at once
eliminated. As regards animals, I need not detail all the early steps
in their progressive evolution. Since some degree of bulk helps to
confer independence of the forces of nature, it is obvious that the
combination of many cells to form a large individual was one neces-
sary step, thus eliminating all single-celled forms from such progress.
Similarly, progress is barred to specialized animals with no blood-
system, like planarian worms; to internal parasites, like tapeworms;
to animals with radial symmetry and consequently no head, like
Of the three highest animal groups—the molluscs, the arthropods,
and the vertebrates —the molluscs advanced least far. One condition
for the later steps in biological progress was land life. The demands
made upon the organism by exposure to air and gravity called forth
biological mechanisms., such as limbs, sense-organs, protective skin,
and sheltered development, which were necessary foundations for
later advance. And the; molluscs have never been able to produce
efficient terrestrial forms: their culmination is in marine types like
squid and octopus.
The arthropods, on the other hand, have scored their greatest
successes on land, with the spiders and especially the insects. Yet the
fossil record reveals a lack of all advance, even in the most successful
types such as ants, for a long time back—certainly during the last
thirty million years, probably during the whole of the Tertiary Epoch.
Even during the shorter of these periods, the mammals were still
evolving rapidly, and man's rise is contained in a fraction of this time*
What was it that cut the insects off from progress? The answer
appears to lie in their breathing mechanism. The land arthropods
have adopted the method of air-tubes or tracheae, branching to
microscopic sixe and conveying gases directly to and from the tissues,
instead of using the dual mechanism of lungs and bloodstream. The
laws of gaseous diffusion are such that respiration by tracheae is
extremely efficient for very small animals, but becomes rapidly less
efficient with increase of size, until it ceases to be of use at a bulk
below that of a house mouse. It is for this reason that no insect has
ever become, by vertebrate standards, even moderately large,