EXTERNAL INFLUENCES AS CAUSES OF VARIATION 235 



be shortened and the organism will move in a curve that will 

 speedily bring it into actual contact. It is noticeable, too, that 

 real contact, being once established, is broken with difficulty. 

 Many lower animals, as in aquaria, coming in contact with a plain 

 surface, move along that surface until they reach a point where 

 side and bottom or where two sides join, and where they can 

 place their bodies in contact with two surfaces. They are likely 

 now to move along the groove formed by the two surfaces until 

 a corner is reached where contact on three sides is possible. 

 Here, if anywhere, the organism will come to rest. It is only 

 that it is " more comfortable " ; that it moved under the molar 

 impulse until it reached a point where further movement and 

 more complete contact were alike impossible. Even higher 

 animals come to the highest state of rest when in contact with 

 foreign bodies on as many sides as possible. 



Effect of contact upon direction of movement, thigmotaxis, 

 or stereotropism. 1 It is a well-known fact that roots growing in 

 running water grow upstream, not downstream, and that many 

 fish at the breeding season are possessed of an irresistible im- 

 pulse to move against the current (rheotaxis). 2 They therefore 

 ascend the strongest currents, leap waterfalls, and surmount 

 every possible obstacle in upstream movements, a passion 

 which ultimately carries them to their breeding grounds in shal- 

 low water. It is at these times that salmon pile themselves up 

 even above the water level and that they will follow any decoy 

 that leads against the current, even into hopeless traps. 



Thus may external agents exert a strongly modifying influ- 

 ence upon such essential activities of living matter as the 

 contractility of protoplasm, resulting in definitely directed move- 

 ments through their control of muscular contraction. As we 

 shall see, contact is not the only influence capable of stimulat- 

 ing contractility of protoplasm and controlling the direction of 

 movement ; on the other hand, muscular tissue is exposed to 

 the exciting influence of a great variety of circumstances both 

 internal and external to the organism, any one of which will 

 induce the characteristic reaction of this sort of tissue, which is 

 contraction. 



1 C. B. Davenport, Experimental Morphology, Part I, p. 105. 2 Ibid. p. 109. 



