234 CAUSES OF VARIATION 



specific rate of vibration, so that when this is reproduced by a 

 vibrating cord or plate, explosion of the substance may occur, 

 lodid of nitrogen is one of these substances which is exploded 

 by a high note." : Living protoplasm is no exception to the 

 general rule that specially unstable compounds are sensitive to 

 contact. 



Effect of contact upon the metabolism of protoplasm. 2 It is a 

 well-known fact that phosphorescence is increased by mechanical 

 irritation. So true is this that the water thrown from the pro- 

 peller wheels of a steamer in tropical regions looks like liquid 

 fire, and a brisk breeze moving over the surface suggests at a 

 distance the white foam of the surf. 



Contact also exerts a strongly stimulating influence upon secre- 

 tions, not only with lower organisms which seek attachments, and 

 the glands of insectivorous plants, but with higher animals as well. 3 



Effect of contact upon movement. The first effect of mechanical 

 disturbance in protoplasm is to check all movement. Minute or- 

 ganisms, tradescantia hairs, etc., cease their protoplasmic motion 

 by the irritation of mounting under the cover glass. In higher 

 plants a sudden jar causes cessation of movement and often a 

 retreat of the protoplasm to one side of the cell so characteristic 

 as to be spoken of as " fright." 



If an amoeba with pseudopodia out is touched or irritated, it 

 immediately assumes the spherical form, and in general the effect 

 of contact is to cause protoplasm to cease motion and assume 

 approximately the spherical form, or, in other words, to occupy 

 the least space possible. But this is to all intents and purposes 

 contraction, and the general principle may be laid down that ex- 

 ternal contact causes contraction, especially noticeable in muscle, 

 which is par excellence the contractile tissue. 



This contraction most commonly, and of necessity, operates 

 at first to draw the organism away from the irritating body 

 (negative thigmotaxis), but if the body be long or large, so that 

 locomotion continues, then the side next the foreign body will 



1 This suggests, as the author observes, the phenomena of hearing. 



2 C. B. Davenport, Experimental Morphology, Part I, pp. 98-99. 



8 As is well known, the heifer that has never produced a calf may be made to 

 give milk merely by persistent manipulation of the udder. 



