CREATION BY EVOLUTION 



is best termed specialization, and the complete set of diver- 

 gent 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, speciali- 

 zation 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; acute- 

 ness 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. Accord- 

 ingly 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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