STRUCTURE AND ENVIRONMENT 25 
90 66 
“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. 
