Harmony and Conflict in the Living World - Alexander F. Skutch [4]
The integuments of animals are wonderfully diverse. Many aquatic and not a few terrestrial creatures enclose themselves in hard shells or carapaces, which may have evolved primarily for protection from predators but at the same time help to insulate animals from the ambience. Insects are covered by their chitinous exoskeletons, penetrated by the tracheal openings through which they breathe. Among vertebrates, the primary integument is a flexible skin, resistant to most substances that are likely to moisten it in an animal's natural environment, constantly renewed as it wears away, and in many animals equipped with sweat glands that help to regulate body temperature, or with chromatophores that by changing its color assimilate to the background and make the wearer less conspicuous to enemies. The scales of fish and reptiles, the hair of mammals, and the feathers of birds give additional protection.
Although every organism from algae and protozoa to trees and the largest vertebrates can regulate the entrance and exit of materials to and from its living cells, only fur and feathers, or subcutaneous fat in certain animals of cold climates, provide effective thermal insulation. Only animals covered with hair or feathers that enclose many minute air spaces can afford the luxury of constant body temperature; for others, the attempt to achieve homeothermy would cost too much energy. By growing a thicker coat of feathers or fur as the climate becomes colder, or depositing more fat beneath their skins, birds and mammals can remain warm and active in air so frigid that all other creatures become dormant or die. They have attained the maximum independence from climatic extremes that animals can achieve without shelters that can be heated or cooled.
Insulation is not only physical but also psychic. We do not doubt that other people feel, and sometimes think, much as we ourselves
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do; and the more intimately we study the lives of other animals, the more certain we become that they, too, are stirred by emotions and are not devoid of thought. But, with certain possible and debatable exceptions, we never have direct, unassailable evidence that other creatures of any kind feel or think; we infer their feelings and thoughts from the way they act, the sounds they emit, their facial expressions. Our psychic insulation is tighter than our physical insulation; the membranes that separate us from our physical environment are but semipermeable, permitting many substances to pass in and out; whatever it may be that shields our minds from direct awareness of the psychic states of other creatures is nearly, if not wholly, impermeable. This insulation makes it possible for one animal to harm another without feeling the consequences.
Although we seldom attribute sociality to lifeless things, they are in fact much more social than living organisms. They seldom enclose themselves in integuments that, like walls, effectively separate them from surrounding materials but freely intermingle when they meet. Rocks and crystals expose their unmodified substance, their naked bodies, to the disintegrative action of air, water, and soil. Gasses of different kinds intermingle, or are absorbed by liquids, with usually no barrier to control the process. Drops of a liquid coalesce when they flow together, one losing its identity in the other. Even solids such as metals slowly diffuse through each other when tightly pressed together. Everywhere in inorganic nature we find readiness to meet and to mingle; no substance appears to be consistently averse to losing its distinctness by union with some other substance. Rarely do we find such aloofness, such stubborn clinging to a separate and insulated existence, as in living things. It is significant that when we wish to waterproof a fabric, or to cover metal or wood with a thin, impermeable pellicle that will shield it from rust or decay, we commonly