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ORGANIC EVOLUTION 



animals, such as fish, frogs, and lizards, can 

 change their skin color to match the back- 

 ground upon which they happen to find 

 themselves. Such concealing coloration is 

 also advantageous to the predator because 

 it can go unobserved until the prey moves 

 into striking range. The positive survival 

 value of such colorations has been shown 

 by experimentation in which the prey with 

 colors most in contrast to the background is 

 always picked up first by the predators. 



Some animals have found survival value 

 in mimicking another species that carries a 

 potent defensive or offensive weapon, such 

 as a venomous snake or a honeybee, for ex- 

 ample. Such well-equipped animals are 

 frequently highly colored, which advertises 

 their danger and warns unsuspecting pred- 

 ators that might take them by mistake. 

 Hence a much lesser creature that might be 

 a choice morsel finds distinct value in re- 

 sembling the more formidable one. Some 

 animals mimic living or dead leaves, and 

 certain butterflies are so faithful in such 

 resemblance that they may go unobserved 

 even by a careful naturalist (Fig. 26-9). 

 Even the veins of the leaves are portrayed 

 by similar markings in the wings of the in- 

 sect. These adaptations and many others 

 undoubtedly have a marked effect on sur- 

 vival, but every such case is open to ques- 

 tion and should be interpreted in the light 

 of actual observations. 



Things of much greater importance than 

 color, body shape, horns, claws, and similar 

 characters are such physiological qualities 

 as ability to resist disease, high or low tem- 

 peratures, desiccation, and so on. Fitness 

 for a specific environment involves many 

 qualities, and it is the product of all of them 

 that makes for survival. It follows that most 

 of them must be favorable, although an ani- 

 mal may survive and be rather successful 

 while carrying along some unfavorable 

 traits. When the weight of the adverse 

 qualities exceeds that of the favorable ones 

 the animal becomes extinct. This has hap- 

 pened to millions of races in the distant 



past and has happened in recent times, as 

 in the case of the passenger pigeon. 



Adaptations tend toward better and bet- 

 ter fitness for the environment which results 

 in specialization. When this has been carried 

 to the extreme, the animal is said to be 

 "overspecialized," a fact which biologists 

 have long considered important in the ex- 

 tinction of certain species and even large 

 groups of animals. Specialization is inevit- 

 able, and all animals are specialized, some 

 much more than others (Fig. 26-10). Land 

 animals, for instance, are adapted for ob- 

 taining oxygen from air, and if the earth 

 were suddenly covered with water they 

 would all be eliminated. This is specializa- 

 tion in a broad sense. As adaptation pro- 

 ceeds, animals become adapted to narrower 

 and narrower limits of their environments 

 and some, such as ruminants, have become 

 adapted to eating grass and probably could 

 not survive if all grasses of the earth were 

 destroyed. Thus survival is intricately 

 linked with consistency of specific environ- 

 ments. 



The more an animal becomes specialized, 

 the more closely it is forced to live in the 

 environment for which it is fitted. There- 

 fore, the more generalized types are less 

 apt to become extinct. For example, omni- 

 vores are more apt to survive under condi- 

 tions where the food is likely to change 

 than strict herbivores or carnivores. If food 

 becomes scarce and the omnivorous animal 

 can no longer exist as a predator it can sub- 

 sist on vegetable matter, or it may devour 

 some of each as the situation demands. 

 This is one of the reasons why the crow is 

 so successful, since it feeds on carrion, 

 grain, leaves, or almost any other form of 

 food. Obviously, then, as animals become 

 more specialized they are endangering 

 their possibility of survival. 



The Irish elk is often used to illustrate 

 this point. While its overspecialized antlers 

 were undoubtedly an important factor in 

 its extinction, they probably were not the 

 whole story. It seems that with each sue- 



