476 GENERAL BIOLOGY 



are alike the end results of a long evolution, with endless 

 adaptations to conditions of life. 



The caddisfly Molanna (fig. 265) "sits close" on a twig, 

 its wings up-rolled about its lifted abdomen, and all the 

 other appendages outspread against the side of a support- 

 ing twig or trunk. So situated on jagged bark or amid 

 the stubs of a twig, it is well nigh undiscoverable. The 

 flying Molanna, settling instantly to this attitude, vanishes, 

 ghost-like, from view. It makes no superfluous motions 

 to hold the eye; actions, attitude and color are in protective 

 accord. But perfected actions, like structures, have to be 

 evolved. Halesus, (fig. 266) sits in a position intermediate 

 between that of Molanna and the more ordinary rest- 

 ing attitude of such insects; and its posture suggests pos- 

 sible stages in the development of Molanna's perfected 

 habit. An ingrained habit may persist, also, like a vestigial 

 organ after it has ceased to be of any use. Perhaps the 

 most familiar illustration of this is the turning round and 

 round of a dog before it lies down. The primitive dog 

 presumably made its lair in the grass, where this was an 

 eminently useful performance. 



Instincts illustrate extreme specialization in the neural 

 mechanism. It is fitted finely to one set of conditions, 

 and is apt to be found wanting when conditions are altered. 

 The moth that flies to the candle flame has left the beaten 

 track of its ancestry, through want of discrimination be- 

 tween stimuli. Another example is furnished by the kinglet 

 (fig. 267) which, being racially unacquainted with the 

 dangerous hooks on the burdock (an imported weed), 

 endeavors to get the seed-eating larvae from the heads for 

 food. Fatal want of discrimination is sometimes displayed 

 toward objects of the normal environment. Thus the flesh 

 fly is stimulated by the odor of the carrion flower and lays 

 eggs upon the plant, where her young on hatching will 



