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EVOLUTION OF FOUR-DIMENSIONAL BEINGS 197
to the infrared or ultraviolet regions of the spectrum because this has survival value
on a particular world. Creatures fulfilling some of these basic trends would be
quite different from us, with various possible symmetries, and they could be as big
as Tyrannosarus or as small as a mouse depending on gravity and other factors.
We might expect intelligent 4-D beings to have digestive systems resembling a
tube structure since we see this so commonly in our world in many different environments.
For example, most Earthly animals above the level of cnidarians and
flatworms have a complete digestive tract—that is, a tube with two openings: a
mouth and an anus. There are obvious advantages of such a system compared to a
gastrovascular cavity, the pouch-like structure with one opening found in flatworms.
For example, with two openings, the food can move in one direction
through the tubular system that can be divided into a series of distinct sections,
each specialized for a different function. A section may be specialized for mechanical
breakdown of large pieces of food, temporary storage, enzymatic digestion,
absorption of the products of digestion, reabsorption of water, and storage of
wastes. The tube is efficient and has great potential for evolutionary modifications
in different environments and for different foods.
As just alluded to, many of our earliest multicelled sea creatures were essentially
tubes that could pump water. As life evolved, this basic topological theme did not
change—the major structural differences involved complex organs attached to the
tube. You and I are still just tubes extending through a body bag filled with "sea water."
In any given dimension, the larger the volume of an animal, the smaller, in proportion,
its exposed surface area. As a consequence, larger organisms (in which the
interior cells are metabolically active) must increase the surfaces over which diffusion
of oxygen and carbon dioxide can occur. These creatures probably must develop a
means for transporting oxygen and carbon dioxide to and from this surface area.
In higher dimensions, the fraction of volume near the surface of a bag-like form
can increase dramatically compared to 3-D forms. This also implies that most of
the blood vessels uniformly distributed through the bag will be close to the surface
too (assuming the creatures have blood). As a result, heat dissipation and movement
of nutrients and oxygen may be strongly affected by the proximity of the volume
to surface. How would this affect the evolution of life? Would primitive 4-D
beings evolve so that they digest on the outer surface, while other organs, like
brains and hearts (whose primary purpose is not nutrient and oxygen acquisition)
are placed deep inside? The outer surface may be digestive and pulmonary, containing
sense organs and orifices for excretion and sex. Is this tendency more likely
as the dimension of the space increases? 1
Here is why the volume of a 4-D animal is more concentrated near the surface
than in 3-D animals. Consider a Z)-dimensional sphere. The volume is