274 



CAUSES OF VARIATION 



any kind, is certainly incapable of exercising anything like in- 

 telligent choice. The characteristic movement must have been 

 due to the specific chemical action of the juices of the meat 

 upon the protoplasm of the muscle fibers of the tentacle, while 

 the paper had no such action and hence no movement followed. 

 This movement and non-movement look like intelligence or 



instinct and have often 

 passed for one or the 

 other to the infinite con- 

 fusion of the subject. 1 



" Lumbrici fcetidi live 

 in the decaying compost 

 of old stables, and prob- 

 ably the chemical nature 

 of certain sub- 

 == stances con- 

 tained in the 

 compost holds 

 them there," 



FIG. 29. Stimulating effect of certain chemicals upon mus- for "when one 

 cular action : a piece of meat laid upon the tentacles of h a jf of the 



the actinian stimulates their action and it is drawn into the 

 mouth. A piece of paper similarly placed is inoperative 



bottom of the 

 box is covered 



with moist blotting paper and the other half with a thin layer 

 of compost, all the worms will gather on the compost side." 

 Decapitated worms behave in the same way, 2 so that the 

 effect is due to a general influence, not to " nerve centers." 

 Loeb says, 3 " I have often placed pieces of lean meat and 

 pieces of fat from the same animal side by side on the window 

 sill, but the fly never failed to lay its eggs on the meat and not 

 on the fat." (He tried, of course without success, to raise larvae 

 in the fat.) " It can easily be shown that larvae of the fly are posi- 

 tively chemotropic towards certain chemical substances which 

 are formed, for instance, in decaying meat or cheese, but which 

 are not formed in fat. The substances in question are probably 

 volatile nitrogenous compounds," and " the chemical effects of 



1 This subject will be pursued further under " Instinct and Reflex Action." 



2 Loeb, Physiology of the Brain, p. 90. 8 Ibid. pp. 186-187. 



