CREATION BY EVOLUTION
is best termed specialization, and the complete set of divergent specializations which characterize the evolution of a whole group, such as the mammals, is called the adaptive radiation of the group. Biological specialization moves always in one direction only and is achieved at the expense of improvement in other directions. What is more, specialization in improving the efficiency of a physical tool, such as a limb or eye, is bound sooner or later to reach a limit. The elephant is pretty close to the limit of size which is possible or at least advantageous for a purely terrestrial animal. The speed of wild horses or antelopes is close to the greatest speed that is possible to a four-legged land animal; acuteness of vision must reach a limit owing to the impossibility of obtaining cells in the retina below a certain size; and so forth.
Thus specialization and adaptive radiation, though they increase immensely the efficiency of life as a whole and enable it to reach its greatest limits in this or that direction, are yet in a sense double-edged. In opening the door to one kind of improvement they close it to other kinds, and in the long run even turn out to be blind alleys, to which positive limits are set. We can easily recognize the limitations of specialization as a method of evolutionary improvement by considering specialization for a parasitic existence. An internal parasite, such as a tapeworm or a malarial parasite, has no need to find or to digest its own food, to move from place to place, or to detect enemies at a distance. Accordingly we find that most internal parasites have no mouth or digestive system, no means of locomotion, or very much reduced means, no well-developed sense-organs. On the other hand, parasites must be specially adapted, for instance, to resist the action of digestive juices or of protective devices in the blood of their host, and especially to enable them to
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