NEW CONCEPTIONS IN SCIENCE 



the light sets up certain chemical reactions which 

 cause the animal to move, just as it sets up a 

 reaction in a photographic plate, or explodes a 

 mixture of hydrogen and chlorine gas. In larger 

 phrase, it is merely a working of positive or neg- 

 ative heliotropism, an attraction or repulsion by 

 the light. 



So with the fly. Certain chemical stimuli from 

 meat cause a fly to lay its eggs. In the fat these 

 stimuli are lacking. They can be produced arti- 

 ficially. In the short-hand jargon of science, it is 

 simply a chemical reaction between certain sub- 

 stances in the skin or sense organs of the fly and 

 the meat, a case of chemo-tactism. 



So some of the flower-like animals. The actin- 

 ians, so much like vegetables that they may be dis- 

 tinguished hardly, will wind their slender tentacles 

 round a bit of crab-meat, but reject a wad of paper, 

 which, to us, tastes just the same. It seems like 

 intelligence to watch it, and the older physiolo- 

 gists found no absurdity in saying that these veg- 

 etable forms "like" meat and "dislike" paper. 

 Yet the actinian, like the fly, merely responds me- 

 chanically to a chemical stimulus. 



And so through all the list. Heat may act as 

 a repellent force; thus, for example, if a moth 

 arrive in the neighborhood of a flame, so that the 

 pushing effect of the heat just balances the pulling 



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