196 Wild Birds 



All animals, moreover, test their surroundings by a method 

 of trial, and error or success, but differ amazingly in the ability 

 to profit by the experiences thus gained. The lower forms act 

 quickly, but with great uniformity; consequently they stumble 

 through life until they finally pay the penalty of their mistakes 

 and inability to learn. The higher animals make the same tests, 

 but apply the knowledge thus gained by varying their conduct 

 to meet the needs of the moment. They control their actions, 

 exercise choice, and display originality, all of which are marks 

 of intelligence. 



In the catalogue of inherited powers, the first place (i) should 

 be reserved for all the general responses or physiological prop- 

 erties of living things, the activities of the cells, upon which 

 the life and growth of the body depend ; to these must be added 

 (2) those direct, and remarkably uniform responses of lower 

 organisms to light, gravity, pressure, and other forms of energy, 

 which determine their orientation or general movements, and 

 may be called trial movements, or tropisms, as well as the closely 

 allied (3) reflex actions, or motor responses, and (4) the still 

 closely related but more complicated and more variable re- 

 sponses, to which the name instinct has been applied. In the 

 voluntary or intelligent response, which is far more variable than 

 instinct, and often very complex, we pass the boundary line be- 

 tween hereditary possession and individual attainment. 



The trial movements, or tropisms, play a great role in the 

 life of the lower animals and plants, and physiologists have 

 shown that many actions formerly ascribed to instinct can be 

 resolved into a series of simpler tropisms (or reflexes, in the wider 

 sense), the number of which is very great. Even the lowest 

 beings in the living world respond to light and other forms of 

 energy in a very definite way, and the kind of response depends 

 not alone upon the nature and intensity of the exciting force, but 

 upon the condition of the animal or plant as well, or the at- 

 tunement of their bodies at a particular time. 



In the nocturnal habits of the earthworm we have an ex- 

 cellent illustration of response to an external force. It remains 

 in its dark burrow by day, and issues forth in the darkness of 



