STRUCTURE AND ENVIRONMENT 25 



"protective resemblance," "mimicry," and "warning coloration" were 

 developed (40). The idea of protective resemblance is as follows: A 

 certain insect is green and lives on green leaves. The natural-selection 

 observer at once theorizes to the effect that the animal is green because, 

 at a time when not all the individuals of that species were green, the 

 birds secured all those not green and left the green ones because they 

 were difficult to see; now therefore only green ones occur. In the case 

 of mimicry, one species of insect (or other animal) resembles another. 

 The theorist finds or thinks that one of them is distasteful to birds and 

 other animals. He further discovers or concludes that the species not 

 having a bad odor or taste is not eaten by enemies because it resembles 

 the distasteful species. The species having the bad odor or taste is the 

 model. The species not having the bad odor or taste is the mimic. 

 The mimic arose and attained its perfection because those individuals of 

 the mimic species which resembled the model species survived. 



In the case of warning coloration, the animal supposed to be dis- 

 tasteful has bright colors. The birds, learning that certain bright 

 colors are associated with bad tastes, avoid such strikingly colored forms. 

 Accordingly, the most brilliantly colored distasteful forms survive. 



More detailed study in recent years has tended to show such specula- 

 tions to be of questionable value. Such ideas must remain matters of 

 speculation at present, because of the difficulty of applying experimental 

 methods to their study. Based on a theory with few facts to support it, 

 and not withstanding critical analysis, the ideas of structural adaptation, 

 including any of the ideas just mentioned, are not a good basis for the 

 organization of a science of ecology. 



The revival of an old idea that animal species arose in places and 

 by methods unknown, and by chance found places to which they were 

 adapted, now constitutes the central idea of the most recent theory of 

 the origin of adaptation and is to be favored as a working hypothesis, 

 because it may be tested experimentally (41). 



Another reason for the inadvisability of attempting to organize 

 ecology on the basis of structure lies in the fact that structural changes 

 resulting from stimulation by the environment are rarely of advantage 

 or disadvantage to the animal, and further that the structure of motile 

 animals is not readily modified by the environment. A considerable 

 number of animals are larger or smaller, lighter or darker, according 

 to conditions surrounding them during development (42), but few 

 biologists see any advantage or disadvantage to the animal in these 

 changes. 



