THE FROG AS A REPRESENTATIVE VERTEBRATE 25 



and are together adapted to the more obvious needs of the organ- 

 ism. Thus, the Hmbs of a vertebrate animal and those of an 

 insect (Fig. 22, p. 42) are structurally adapted to function in 

 a manner that is advantageous for the life of their possessor. A 

 famihar example of what is generally regarded as an adaptive fea- 

 ture is the coloration and bodily shapes of many animals, notably 

 birds and insects, which so resemble the surroundings that the 

 animal seems likely to escape the observation of many enemies 

 (Fig. 8). The behavior of an animal as a whole, as shown in 

 actions like eluding an enemy or seizing a victim, is, therefore, 

 adaptive insofar as it tends to preserve the life of the individual 

 under the normal conditions of its environment. Such adaptation 

 is not perfect, but only sufficient for the needs of the particular 

 case. The snake that swallows a hen's egg profits by the experi- 

 ence, but should it swallow a china nest-egg it might die of indi- 

 gestion. Frogs and toads snap at objects moving in the air near 

 their heads. In nature, motion of this sort is almost invariably a 

 sign of something good to eat. Behavior in frogs and toads is 

 adapted accordingly. If the moving object happens to be a fish- 

 hook, the animal may lose its life. 



In general, it may be said that the behavior of animals is 

 adapted to the conditions which they and their ancestors have 

 commonly experienced. They are not adapted to untried situa- 

 tions, unless by accident. When animals come in contact with such 

 situations, .one of two things seems likely to happen: either the 

 species is exterminated through its inabiUty to cope with these 

 conditions; or, after a period of wholesale destruction, the sur- 

 vivors become adapted. This adaptation may involve the devel- 

 opment of new structures as well as new functions and modes of 

 behavior, and hence may effect what is clearly an evolutionary 

 modification. By such modifications in relation to changes of the 

 environment, it is beheved that species have reached their present 

 state of fitness. 



Charles Darwin (1809-1882) designated as '' Natural Selec- 

 tion " this action of the environment whereby the indi\dduals of a 

 species are selected by nature according to their abiUty to meet 

 the demands of an intense struggle for life. Granting the reality of 

 adaptation as a widespread phenomenon among animals and plants, 

 his theory remains the best scientific explanation of the manner 

 in which such fitness has reached its present degree of excellence. 



