222 Education through Nature 



eyes. The compound eyes of arthropods are appar- 

 ently aggregations of simple eyes. 



The simplest ear is perhaps that of the ptenophores. 

 It is hardly more than a simple bag, an infolding of 

 the outer cuticle, filled with a fluid containing one or 

 more ear-stones, or otholiths. As in the case of other 

 sense-organs, the ear becomes more perfect as we 

 ascend in the scale. In fishes there is no external ear, 

 but the internal ear is well developed. In the frog 

 the ear-drum or tympanum is naked. 



Development in the animal series seems to be accom- 

 panied by a development of sense-organs correspond- 

 ing to development of a central nervous system. The 

 intelligence of the animal is proportional to its power 

 of recognizing external objects and influences through 

 the sense-organs, and of interpreting them by cerebral 

 activity, so as to adjust itself to those influences. 

 Some of that adjustment consists in movement. 



Reflex Action and Instinct. The foundation of the 

 intellectual part of animals is sensation and motion. 

 An immediate motor response to a sense stimulation 

 is reflex action. Instinctive action is hardly more 

 than a very complex reflex act, in which the response 

 may have been considerably delayed. 



Thus the reflex acts of the lower forms pass gradu- 

 ally into the instinctive acts of the higher. Instinctive 

 acts, too, are not separated by a hard or fast line from 

 intelligent acts. 



