))/(j//v ///(• froccs.u's indiiciiKj locoiiioliou arc equal in 

 both lialvrs of the central ncn-oiis system, and the 

 tension of the syniinctrical ntiisclcs hein;/ equal, the 

 animal moves in as straight a line as the imperfec- 

 tions of its locomotor apparatus permit. If, however, 

 the velocity of chemical reactions in one side of the 

 body, c.j/.. in one eye of the insect, is increased, the 

 physiological symmetry of both sides of the brain and 

 as a consequence the equality of tension of the sym- 

 metrical muscles no longer e.vist. The muscles con- 

 nected ti'lth the more strongly illuminated eye are 

 thrown into a stronger tension, and if new impulses 

 for locomotion originate in the central nen^ous sys- 

 tem, they will no longer produce an equal response 

 in the symmetrical muscles, but a stronger one in the 

 muscles turning the head and body of the animal to 

 the source of light. The animal «•('// thus be com- 

 pelled to change the direction of its motion and to 

 turn to the source of light. . . (Loel) 1918). 



The idea that all instinctive activities of organ- 

 isms were forced and invariable responses to en- 

 vironmental factors met many objections. H. S. Jen- 

 nings (1906) pointed out that many Protozoa are 

 asymmetrical in body structure and hence could not 

 lend support to the tonus theory. Furthermore, the 

 movements and responses of many organisms to en- 

 vironmental stimuli were not stereotyped, but random 

 in nature : of a trial and error sort. Although much 

 of Loeb's theory has been disproven experimentally 

 and appears untenable on the basis of observations 

 of animal activities under natural conditions, it crys- 

 tallized the need for objective analysis and interpre- 

 tation of animal behavior, and the avoidance of teleo- 

 logical and anthropomorphic explanations. The study 

 of orienting responses of organisms is of utmost 

 ecological significance since it is largely by means of 

 such responses that organisms find their proper and 

 favorable habitats. 



Prejerendum 



The behavior responses of animals and their 

 orientation in respect to most environmental factors 

 can be tested experimentally, and results thus ob- 

 tained correlated with the animal's behavior under 

 natural conditions. There is a variety of procedures 

 and equipment suitable to these purposes (Shelford 

 1929, Warden, Jenkins, and Warner, I, 1935) and 

 there is distinct value in verifying field observations 

 with e.xperimental analyses. 



When the number of favorable responses at each 

 unit intensity of an environmental factor is plotted 

 against the entire range of that environmental factor, 

 the usual result is a normal or Gaussian curve. The 

 maximum number of responses normally occurs near 

 the center of the range, with a progressive reduction 



in nniui)cr toward each cxtn-me. \u extension in 

 eacii direction from the i)eak of the rcs|)onscs to in- 

 clude ."^O, 2^, or some smaller percentage of tiie total 

 responses is called the preferendum for that animal 

 or griiuii of animals. 



IniKilr hihniiiir 



.Much of the behavior of organisms is deter- 

 mined by heredity and is characteristic of the species 

 in its ])roper environment. This behavior may be evi- 

 dent at birth or it may not develoj) until the nervous 

 .system, including both the receptor and eflfector mech- 

 anisms, is fully matured. Such innate behavior is of 

 various degrees of complexity. A reflex is a quick, 

 automatic response of a single organ or organ system 

 to a simjile stimulus: for instance, the knee jerk in 

 man. Tropisms, taxes, and kineses may involve a 

 series of reflexes and represent a higher level of in- 

 tegration. An instinct, or inherited behavior pattern, 

 is a complex fixed behavior that is activated, more or 

 less automatically, when the animal is presented with 

 the proper stimulus (Thorpe 1951). 



The anatomical basis for these various grades of 

 iiehavior lies in the structure of the nervous system 

 and especially, in higher types of animals, in the in- 

 terarrangement of neurones and synapses with each 

 other and in the neural pathways that become estab- 

 lished. Behavior patterns become elaborated through 

 evolution, are as subject to mutation as any struc- 

 tural part of the body, and are a means whereby ani- 

 mals respond advantageously to the various factors 

 in their normal environment. 



Stimuli 



Before an action will take place the nervous 

 mechanism must be released by the reception of a 

 stimulus. Stimuli may be either external or internal 

 to the organism. Protoplasm is sensitive to any kind 

 of stimulation, provided it is intense enough. In 

 higher organisms, however, specialized tissues have 

 become particularly sensitive to one kind of stimulus, 

 and these tissues, or sense organs, are called recep- 

 tors. There are several forms of receptors : photo- 

 receptors, phono-receptors. mechano-receptors, 

 chemo-receptors, thermo-receptors. and stato-recep- 

 tors. Not all types of receptors are present in all 

 organisms, and the structure and effectiveness of 

 those present varies from one kind of animal to an- 

 other. The efficiency of the receptor mechanisms is 

 important, as they largely determine the environ- 

 mental factors to which the animal will respond and 

 the degree of sensitivity involved. 



Stimuli may be internal, and derive either from 



The general nature of responses 



